Varies on the company. Meta does a relatively thorough background check as of this last year. They verify past employers and other background check stuff using HireRight. If you accept an offer in their NYC office, they do a more extensive background check for some reason. Maybe that kind of background check is outlawed in CA and WA but isn't in NY. I have no idea. I only know cause I've gone through the team match with them. (Don't recommend - never received an offer even after being in team match for 9 months)

Some of these systems are relatively automated but almost every company uses a third party. Very few employers will do the phone calls themselves. If you worked at small startups, they will often call the CEO's personal phone number. So, you'll need to tell them to pick up the phone. They will verify things like title and dates you were employed.

I've had five different employer in the valley. Personal references never got called - which I always found funny. Some places wanted references before they even gave you an offer! Those guys are a waste of time, btw. They always sent insane low ball offers. Like if you said your minimum was $200k/yr salary in the first interview, they'd send you an offer for $150k/yr. I was floored at such idiotic behavior. You went from having little chance because I was using you for practice anyway to literally zero.