Environment variables are global variables that are accessible from every system running in the OS (Operating System).
There are multiple forms of environmental variables that can be used in a java application.
Those accessible to any project in the operating system and those accessible to a specific java application
The 2 links below describe what environment variables are and how to update them.
You can read through the following example to see how you can access the environment variables from a Java application.
A typical use case for environment variables is to store the paths to certain directories, such as paths to folders holding SDKs.
java-dotenv is a package that can be imported into a java project to assist with simplyfying using environment variables in applications.
Documentation for its use can be found here.
deployment configurations refers to different build environments, each with their own set of variables that can be used when the java application is built using those environments.
For example: If you have an Api for your development environment and an Api for Production, you can make use of gradle build types to inform your application which Api it should communicate with in a specific build environment.
This automates the process of changing environments. So instead of having to manually change the Url of your Api to toggle between development and production, you can create a debug and release build type. So changing the build environment will automatically point to the correct version of the Api you are using.
This documentation describles how to create different gradle files for different situations in an Android project.
Android Apps can however be created using Java, so implementing gradle rules in a java application will follow the same format as the documentation above describes.