Putting their butt in your face: they want you to lick it, as their mother would. Obviously, don’t do that, but instead, give their haunches and above-the-tail backside a good scritch
Unless the cat is in the same position pictured in op, because in my experience their tail will go up and you will have a butthole directly in your face 😂
https://vetexplainspets.com/why-do-cats-lick-other-cats-butts/
One of the main reasons cats lick each other’s butts is for grooming purposes. Cats are meticulous groomers and spend a significant amount of time cleaning themselves and their fellow feline companions. Licking each other’s butts allows cats to clean hard-to-reach areas and helps maintain their hygiene.
The cat looked unhappily back at its butt. The problems resulting from concurrently wearing a built-in fur coat, pooping, and not having any hands to operate toilet paper were clearly not appreciated by the human.