I' like to sink my teeth into that question! They go to other dentists, although for a cleaning they would have their dental hygienist do that. What time is it? ToothHurty, time to see my dentist!
I asked my dentist who has been practicing for over 40 years and he said he has one of his dentists in his practice do the work. He doesn't instruct them, but let's the dentist do the work and he keeps out of it.
I used to know a dental assistant and she said they go to other dentists.