How to get acces to offline YouTube videos downloaded to my Android 9 Cellphone? I use to download youtube videos so I can watch then offline. But I don't like the "YouTube video player". So, how could I have access to the offline youtube videos so I can open them using my video player of choice?

I have a Android 9 cell phone.
The offline videos in YouTube app can only be played within YouTube. You cannot play them in an external player.

Your alternate option is to download those videos as MP4 files and then playing them in your favorite player. But do remember that downloading videos are against YouTube's terms and you won't find more help on that topic over here.