For example, take this famous position from Kramnik vs. Deep Fritz, 2005:
[FEN "5N1k/6p1/7p/4P3/pp2Q3/4q3/1P4PP/2b4K w - - 0 1"]
Obviously White wins with Qh7#, and equally obviously, there are no possible better moves (since it's impossible to have a mate in 0).
If I let an engine run on this position, what does it do after finding the mate in 1? There's presumably nothing else to do since it has already found the best move. Does it just exit, then?
I tried running Stockfish 13 on this position, and it reaches depth 99/99 instantly, but I can't tell what it's doing afterwards.