Loading environment variables from a .env file if it exists is no different between development and production.

You just set the variables at startup in prod, and in the file on development (probably mostly because you want to change them all the time).

The devil's in the details.

There's an enormous difference between "I'm going to use a dev-friendly way to manage and swap out all the different configurations that I might want to plug into the app while I'm testing it, but still plug them in using the same mechanism", which is basically what you get with dotenv files, and, "I'm going to configure with environment variables in production and JSON files in development," which is another thing I often see people doing.

Ops person here. .env files at best should be injected into Developer environment by IDE/Editor. Many IDE/editors have this feature along with Docker. If that's not an option (Thanks JetBrains/Visual Studio), second thing is something like Python-dotenv where library loads .env file at startup, injects them into environment variables and rest of the code is configured to use environment variables. This behavior could even be environment variable controlled and should not error if no .env file is present.