Does the seller stay in the building at all on closing day, or do they have to leave immediately?

There's no default rule here — it depends entirely on what the purchase agreement, or a separate transition arrangement, actually says. Some deals have the seller vacate the premises as soon as possession passes to the buyer at closing. Others deliberately build in a short transition period where the seller, or a principal of the seller, stays on briefly to introduce the buyer to staff, customers, and suppliers, sometimes formalized through a separate consulting or short-term employment arrangement rather than left informal.

Because there's no automatic answer, this is something to negotiate and document clearly well before closing day, not something to assume based on general expectations either way. If a transition period matters to you as a buyer — or if you're a seller who wants (or wants to avoid) staying involved after closing — get that spelled out in writing, including how long it lasts and what the seller's role actually is during it, rather than leaving it to be worked out informally on the day itself.