How does the .env.php has to look?

On Forge, you have to manually add your environment variables. Its located under site detail->environment. You have to set you database info there.

I did this for the DB variables.

But what about my questions?

The .env file is only use for locally. its not being commited.

I'm assuming you are using L4, as L5 doesn't have a .env.php file..

1) It depends on your environment checking. L4 by default checks your servers host name, not the value of APP_ENV

2) You can add environment variables using forge as @fraserk has mentioned. If you prefer to do it the manual .env.php way then yes you will have to upload this yourself with ssh/sftp etc. The idea is this file doesn't exist in your git repo, because it contains all your api keys, and passwords etc.

3) It's just an array of data that you return, something like this:

<?php

return [
     'DB_PASSWORD' => 'np9c8y2bc9nt2px8d',

     'DB_USER' => 'forge',

     'DB_NAME' => 'mydatabase',

     // And so on...
];