Sometimes when a cat grooms another cat, it’s nice. A cat helping another cat out, cleaning those hard-to-get spots on the back of the neck with a friendly lick.
Other times? It’s more like, “Hey, I want that nice nap spot, and I’m going to annoy you until you give it to me.”
Cats also do it to infantalize each other. “you are a filthy kitten and I’m doing this because you’re my little bitch” which cats also play the “I’m just a cute wittle meow meow pls help” card so its never clear which or how they are manipulating each other than the complexity and range of their interactions.