Plasterer bees

Happened upon a swarm of solitary bees mating beside a path through a forest. Near the swarm, I saw many holes in the sandy ground, but since there were also a lot of ants around, I can't be certain that the bees emerged from there. The first photo shows one female with multiple males, while the other photos (and this Youtube video: https://www.youtube.com/watch?v=067H2RyY6Ck) show the same female with just one of the males.