I didn't find how I can get the file from the Uri.
There is no file. ACTION_OPEN_DOCUMENT and ACTION_GET_CONTENT do not open a file. They open a document. That document might be a file. It might not. That Uri might point to:
A local file on external storage
A local file on internal storage for the other app
A local file on removable storage
A local file that is encrypted and needs to be decrypted on the fly
A stream of bytes held in a BLOB column in a database
A piece of content that needs to be downloaded by the other app first
...and so on
How can I get the real path of the file ?
If you wish to only accept files, integrate a file chooser library instead of using ACTION_OPEN_DOCUMENT or ACTION_GET_CONTENT. Just bear in mind that filesystem access to external storage is limited on Android 10+.
If you use ACTION_GET_CONTENT, and the scheme of the Uri that you get is file, then getPath() will be a filesystem path.
Otherwise, you need to understand that you have no idea where the document is coming from, and stop thinking in terms of "real path of the file". Use ContentResolver and openInputStream() to make a copy of the content to some file that you control, then work with that file.