Orbs

Can someone provide a detailed description of what causes orbs to
appear on photographs, please? I've looked on the web and all
I find is either idiots talking about photographing fairies, aliens, ghosts
or whatever. Other than that I find single line answers, like "out of
focus specks of dust"

I'd like something a bit more detail;ed and helpful than that. Eg,
information about lighting conditions, type of camera and so on that
would produce the effect.

TIA