What's the "inside" of the butt? On a scale of 'where the sun don't shine while clenching cheeks' to 'where its water tight'?

Any what part of someone's body can get how close?

Would you do a butt job? Like where you're hotdogging your penis between their cheeks? You'd be lubed up but you wouldn't actually be entering the anus, you'd be rubbing against it orthogonal to the hole.