I came in Canada on a tourist visa and want to travel from Vancouver to Seattle by bus because it's very cheap for me. I have a USA B1/B2 visa as well, so where do they do immigration for US while entering there on Bus? Because I have a tourist visas and I need border exit stamps on my Indian passport.
Done this trip many times.
As you get to the border, the bus company pulls over. You take your bags off with you, go through security and passport control. They check your visa etc.
Then you all pile back onto the bus, and continue on your way.
Usually takes about 25 min for a full bus, and is included in the travel time.