Speaking to a customs official is very rare, I've only had that happen once.
Do most Americans get questioned at their own border? I do, but I'd assumed that was only for "aliens".
Similar thing although with a few more questions entering Canada. Usually it's just, do you have anything to declare - which seems like a perfectly acceptable thing to ask.
'Is this really you?'
'Yes'
'Okay, thanks.'
You have to hand your receipt from the machines to an agent who may or may not decide to question you at that point. After baggage claim, you may or may not get stopped by a CBP agent who decides to question you based on their "discretion". (At some airports, these two checks are consolidated and done once, after baggage claim).
Some people are stopped for these checks more frequently than others.
As a 20s non-white guy I'm never bothered, so maybe I'm just off-base here.
That stopped happening when I reached my 40s.
Bringing a citrus fruit from most countries, various other fresh foods, counterfeit goods, etc.