Use .env files in your Unity projects.

npm

https://docs.unity3d.com/Packages/com.unity.package-manager-ui@2.0/manual/index.html

{
 "dependencies": {
 "xyz.candycoded.env": "https://github.com/CandyCoded/env.git#v1.1.5",
 ...
 }
}

{
 "dependencies": {
 "xyz.candycoded.env": "1.1.5",
 ...
 },
 "scopedRegistries": [
 {
 "name": "candycoded",
 "url": "https://registry.npmjs.com",
 "scopes": ["xyz.candycoded"]
 }
 ]
}

Create a .env file at the root of your project, outside of the Assets/ folder, and paste the following content:

DEBUG=true

Or use the Editor panel found by navigating to Window > CandyCoded > Environment File Editor.

Note: Don't forget to add .env to your .gitignore file before committing any changes!

Now you can reference the variables and their values with the key specified in the .env file. Supported value types are string, bool, double, float, and int.

if (env.TryParseEnvironmentVariable("DEBUG", out bool isDebug))
{
 Debug.Log($"Debug Mode is: {(isDebug ? "ON" : "OFF")}");
}