In driving across I’ve experienced both. We’ve been waved right through and we’ve been made to get out and go through the whole immigration/customs process. Last October I took a tour bus of 44 people to Rosarito for the day. We had to get off the bus and go through their process. PITA. This past February I was in a van with 9 people and we were waved right through.
If you’ve parked at the border or arrived by trolley, bus or drop off, you use a different crossing than by vehicle. It’s on the far east side of the port of entry. Getting off the trolley it’s super simple, you just kind of walk up the alley south of McDonalds, follow everyone to the right and go through the revolving gate. After that there is only one way to go, into the immigration line. That’s been the process since last September. It’s also a PITA but not so bad if there aren’t many people ahead of you. Mexican nationals can bypass the immigration line, but they do have to go through the customs line.