I Have an Answer Sort Of
I downloaded a lot of players and never set a default so i could test, this is the screen that pops up when i select an .mp4:
When I select the 'Movies' player, the bar on the bottom goes dark like so, look beneath the red marks i made, there are 4 barely visible dots. I believe this is called Dark Mode, the bar is still there just barely visible.
The problem is there is no menu in the player and no program on my tab called 'Movies'. It's either stock, or came with a file manager.