The brushes usually don't go all the way to the sides of the vacuum, that's why many of them have an additional circular brush on the side to get into the corners and scoop the dirt to the big brushes in the middle.

No need for edges, also makes navigating through narrow things easier.

Mostly circular, some have slightly asymmetric shapes like Eufy e28. The mid range and higher typically have an extending arm that handles edges or corners