Mine loved when I did it back to her, she would cuddle me hard but only with me. I miss that little shit
They have glands there.
Human must smell like cat (it’s their territory), cat musn’t smell like human. Ieuw.
They’re not showing affection, they’re marking you.
It’s both. Domestic cats are social creatures. They possess at least rudimentary Theory of the Mind, so, understand that we are other beings, not objects. They identify their family unit through sharing and intermingling scent.
Cats are very scent-based, especially up close, where their eyesight is poor. This is observable when a cat displays aggression towards a “friend” who has the smell of the vet or a bath. From far away they look right but up close, they smell wrong - it’s like a feline uncanny valley.