Do the dogs in this universe enjoy things like being pet? Is scratching behind someone's ear or rubbing their belly a part of close friendships and relationships for them like back rubs and playing with hair are for humans irl?

I suspect many humans enjoy being petted too.

The displays of affection in the dogverse, platonic or romantic, aren't that different from our own. Ear scratches are a popular and common gesture, I guess if you're close enough to play with someone's hair you could also scratch behind their ears if they were a dog, you know? Then again ears are sensitive and used in communication, so maybe there's a slightly higher degree of trust required.