They don't swim. When they are in a non-flocculated state, bouyancy tends to suspend them and brownian motion tends to keep them that way, i.e. randomly dispersed throughout the wort. Later, convection currents from the heat produced by fermentation keep the yeast moving, and the CO2 produced tends to bounce them around too.
To piggyback on what @VikeMan said, yeast do not possess any moving appendages to be able to move about on their own.
Hahahaahahaha can’t wait for the next grandson visit. I’ll b sure to tell them why the yeast moves, Jack, style.
