Why are there round white spots in the photos taken?

Because there are countless dusts and dusts floating in the air, during flash photography:

1) The light of the flash lamp not only illuminates the object to be photographed, but also illuminates the dusts and dusts floating in the air.

2) The dust floating in the air and the light of the dust are reflected to the lens picture together with the light of the photographed object.

therefore, circular white spots can be seen on the photo screen, but this phenomenon depends on the lens size.