I found a solution! The problem was not in the "downloader_video_fnc" function, but in the "MessageHandler"

The correct MessageHandler for concurrency is

application.add_handler(MessageHandler(filters=(filters.VIDEO & filters.User(username="@XXXXXXXX")), callback=downloader_video_fnc, block=False))

block (bool, optional) – Determines whether the return value of the callback should be awaited before processing the next handler in telegram.ext.Application.process_update(). Defaults to True. MessageHandler manual