Since it clearly is not "kissing", what is the actual behavioral basis for a dog licking your face?

Puppies born in the wild will lick the face of their mother to encourage regurgitation after a hunt.
Domesticated dogs retain this natural instinct even when adults, the pooch wants its dinner!

It’s also used as a salute. “Whassup”. It doesn’t even have to involve contact.

Year’s ago I started greeting my best friend’s dog by bending down for a kiss. Now when I arrive he goes crazy and won’t leave me alone until we properly greet.

We do “air kisses” like 5th Avenue socialites. I bow down, and he licks the air two feet from my face.

It depends. It can be affectionate, or a greeting, or it can be a sign of submission or it can be them trying to tell you that they want/need something.

They like the taste of the salt in your sweat.

I reject the food idea since my dog can have a bowl of food and still be affectionate. But I think they see it as an action of grooming which means a sort of affection to them. Something they are taught early on from a new mom who will lick them from top to bottom. And since people will often kiss their dog or pet them, than they are simply reciprocating and may see this as a way of greeting. .

I dog sat two adult pitbulls once, and they basically made out with each other. They canted their open mouths at a 45–90 degree angle, and their tongues were all over the insides of their mouths. I witnessed this nearly every day for a month.

There are several reasons a dog may lick your face. Appeasement is one, so when you’re being licked, the dog could be fearful of you. However, dogs lick the faces of their friends simply to strengthen the mutual bond. Presumably, it feels good.

Young puppies will also lick faces to prompt an adult dog to regurgitate food.

‘A dog does not know what “kiss” or “affection” means.’

^ Would you share your research on this, or is it simply a belief of yours? I’d be interested to understand why you consider the ability to feel affection a purely human trait. Apart from this being illogical due to all mammal’s similarities in basic brain structure, it also seems like an argument of about half a century ago. Have you read an outdated book on this, by any chance?