What happens if two people update the same cell at the same time?

The same applies with more saves.

If user A makes a change, saves, makes a change, saves and user B has the sheet open and then makes a change and saves, B wins.

Last save is last. The version history will show each save.

When working with others, watch for "refresh", and refresh/save often.

Some form of communication may be useful - chat, voice.

Before making wholesale changes (like adding/deleting rows), best to warn your collaborators.

Craig