The files are read inside this browser tab and never sent anywhere, and values are drawn as black bars so you can share your screen. Still, for production secrets you can paste the file with the values deleted: the check only needs the names.
Variables in .env.example that your .env lacks, empty values, placeholders you forgot to fill in (changeme, your_key_here, <token>, undefined), extra variables, duplicates, and formatting that other tools read differently.
dotenv, version 15 and later, starts a comment at any #, so PASSWORD=abc#123 is read as abc. Wrap the value in quotes and the whole thing is kept.
The last one, as in dotenv. The tool tells you whether the two values are the same, and which line wins, without showing the values.
dotenv ignores them, but docker run --env-file stops with an error on a space before =, and keeps a space after = as part of the value. The same file can work locally and fail in a container.