feat: Reorganize environment variables documentation for clarity

sources/platform/actors/development/programming_interface/environment_variables.md

@@ -1,40 +1,34 @@
 ---
 title: Actor environment variables
-description: Learn how to provide your Actor with context that determines its behavior through a plethora of pre-defined environment variables offered by the Apify SDK.
+description: Learn how Actors receive runtime context from system environment variables set by the Apify platform and custom variables you define.
 slug: /actors/development/programming-interface/environment-variables
 sidebar_position: 3
 sidebar_label: Environment variables
 ---
 
-**Learn how to provide your Actor with context that determines its behavior through a plethora of pre-defined environment variables set by the Apify platform.**
-
 import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';
 
----
-
-## How to use environment variables in an Actor
-
-You can set up environment variables for your Actor in two ways:
+Actor runs get their environment variables from two sources:
 
-- [Set up environment variables in `actor.json`](#set-up-environment-variables-in-actorjson)
-- [Set up environment variables in Apify Console](#set-up-environment-variables-in-apify-console)
+- [System environment variables](#system-environment-variables) - set automatically by the Apify platform for each Actor run.
+- [Custom environment variables](#custom-environment-variables) - defined by the Actor owner, either in `.actor/actor.json` or in Apify Console.
 
-:::info Environment variable precedence
-
-Your local `.actor/actor.json` file overrides variables set in Apify Console. To use Console variables, remove the `environmentVariables` key from the local file.
-
-:::
+By default, both kinds of variables are passed only to the Actor **run**, not to the **build**. For variables passed to the build process (Docker build arguments), see [Build-time environment variables](#build-time-environment-variables).
 
 Check out how you can [access environment variables in Actors](#access-environment-variables).
 
 ## System environment variables
 
 Apify sets several system environment variables for each Actor run. These variables provide essential context and information about the Actor's execution environment.
 
-Here's a table of key system environment variables:
+:::info Runtime only
 
-| Environment Variable | Description |
+System variables apply only to Actor runs and are never passed to builds - not even when **Apply environment variables also to the build process** is enabled in **Code** > **Environment variables**. That option forwards only your custom variables.
+
+:::
+
+| Environment variable | Description |
 | -------------------- | ----------- |
 | `ACTOR_ID` | ID of the Actor. |
 | `ACTOR_FULL_NAME` | Full technical name of the Actor, in the format `owner-username/actor-name`. |
@@ -83,9 +77,19 @@ All date-related variables use the UTC timezone and are in [ISO 8601](https://en
 :::
 <!-- vale Microsoft.RangeFormat = YES -->
 
-## Set up environment variables in `actor.json`
+## Custom environment variables
+
+Actor owners can define custom environment variables to pass additional configuration to their Actors.
+
+- [Define in `actor.json`](#define-in-actorjson)
+- [Define in Apify Console](#define-in-apify-console)
+
+Your local `.actor/actor.json` file overrides variables set in Apify Console.
+To use Console variables, remove the `environmentVariables` key from the local file.
 
-Actor owners can define custom environment variables in `.actor/actor.json`. All keys from `environmentVariables` will be set as environment variables into the Apify platform after you push Actor to Apify.
+### Define in `actor.json`
+
+Actor owners can define custom environment variables in [`.actor/actor.json`](/platform/actors/development/actor-definition/actor-json). All keys from `environmentVariables` will be set as environment variables into the Apify platform after you push Actor to Apify.
 
 ```json
 {
@@ -94,44 +98,39 @@ Actor owners can define custom environment variables in `.actor/actor.json`. All
   "version": "0.1",
   "buildTag": "latest",
   "environmentVariables": {
-    "MYSQL_USER": "my_username",
+    "MYSQL_USER": "my_username"
   }
 }
 ```
 
 :::info Git-workflow with actor.json
 
-Be aware that if you define `environmentVariables` in `.actor/actor.json`, it only works with [Apify CLI](/cli). If you use a Git workflow for Actor development, the environment variables will not be set from `.actor/actor.json` and you need to define them in Apify Console.
+If you define `environmentVariables` in `.actor/actor.json`, it only works with [Apify CLI](/cli). If you use a Git workflow for Actor development, the environment variables will not be set from `.actor/actor.json` and you need to define them in Apify Console.
 
 :::
 
-## Set up environment variables in Apify Console
+### Define in Apify Console
 
-Actor owners can define custom environment variables to pass additional configuration to their Actors. To set custom variables:
+To set custom variables in Apify Console:
 
-1. Go to your Actor's **Source** page in Apify Console
+1. Go to your Actor's **Source** page in Apify Console.
 
 1. Navigate to the **Environment variables** section.
 
 1. Add your custom variables.
 
-For sensitive data like API keys or passwords, enable the **Secret** option. This will encrypt the value and redact it from logs to prevent accidental exposure.
+### Secure environment variables
+
+For sensitive data such as API keys or passwords, enable the **Secret** option when defining a variable in Apify Console. Secret values are encrypted at rest and redacted from Actor run logs to prevent accidental exposure.
 
 :::caution Visibility of environment variables in public Actors
 
-When you [publish your Actor](/platform/actors/publishing/publish), environment variables not marked as **Secret** in Apify Console are visible to anyone on the Actor detail page alongside the source code. Enable **Hide source files from Actor detail** in the Actor's **Settings** to hide both.
+When you [publish your Actor](/platform/actors/publishing/publish), environment variables not marked as **Secret** are visible to anyone on the Actor detail page alongside the source code. Enable **Hide source files from Actor detail** in the Actor's **Settings** to hide both.
 
 Secret environment variables are never exposed on the Actor detail page regardless of this setting. Always mark sensitive values as **Secret**.
 
 :::
 
-:::info Build-time variables
-
-Once you start a build, you cannot change its environment variables. To use different variables, you must create a new build.
-
-Learn more in [Builds](../builds_and_runs/builds.md).
-:::
-
 ## Access environment variables
 
 You can access environment variables in your code as follows:
@@ -178,9 +177,10 @@ async def main():
 </TabItem>
 </Tabs>
 
-## Use the `Configuration` class
 
-For more convenient access to Actor configuration, use the [`Configuration`](/sdk/js/reference/class/Configuration) class
+### Use the `Configuration` class
+
+For more convenient access to Actor configuration, use the [`Configuration`](/sdk/js/reference/class/Configuration) class.
 
 <Tabs groupId="main">
 <TabItem value="JavaScript" label="JavaScript">
@@ -219,19 +219,28 @@ async def main():
 </TabItem>
 </Tabs>
 
+
 ## Build-time environment variables
 
-You can also use environment variables during the Actor's build process. In this case, they function as Docker build arguments. To use them in your Dockerfile, include `ARG` instruction:
+The environment variables described above apply only to Actor **runs**. To make a variable available during the Actor's **build** process, you need to opt in explicitly. In this case, the variables function as Docker build arguments. To use them in your Dockerfile, include the `ARG` instruction:
 
 ```docker
 ARG MY_BUILD_VARIABLE
 RUN echo $MY_BUILD_VARIABLE
 ```
 
+To pass your user-defined environment variables to the build, enable **Apply environment variables also to the build process** in your Actor's **Code** > **Environment variables** section in Apify Console.
+
+:::caution Only user-defined variables are passed to the build
+
+Even with **Apply environment variables also to the build process** enabled, only the variables you define in the Actor's **Environment variables** section are forwarded to the build. The Apify [system environment variables](#system-environment-variables) (such as `ACTOR_RUN_ID`, `APIFY_TOKEN`, or `ACTOR_DEFAULT_DATASET_ID`) are never available at build time, since the build is not associated with a specific run.
+
+:::
+
 :::caution Variables set during the build
 
 Build-time environment variables are not suitable for secrets, as they are not encrypted.
 
 :::
 
-By leveraging environment variables effectively, you can create more flexible and configurable Actors that adapt to different execution contexts and user requirements.
+Once a build starts, its environment variables are frozen into that build's Docker image. To change them, you need to create a new build. Learn more in [Builds](../builds_and_runs/builds.md).