Auntie stated it all perfectly- I'll just add what happened with my son (HFA) this past year in kindy. He was in a small half-day kindy class (mainstreamed). All the ASD kids were in this class, so there was a mainstream kindy teacher, and ASD teacher, and some floating aides. After the mainstream class concluded, DS went to his contained ASD classroom where the teacher went over the lessons they went through in the morning and they simultaneously worked on social skills. He had ST, OT, and APE pullouts once a week. They were supposed to organize these pullouts in the afternoon so as to maximize time in the mainstream setting.
Next year, because the district has moved to entirely full-day kindy, the ASD kids will be mainstreamed again in one classroom, but with more pull-outs for social skills and other services.
Like auntie said it can vary considerably. Our school is an "ASD site" so there are a lot of kids with ASD around and they have a pretty well established program set up.
There is a kid in my son's school who has been with my son in EI since he was a toddler. He's going into third grade now and is actually leaving to go to a private parochial school- there are no services left on his IEP. When he was young he was a runner- I chaperoned a bowling trip with him and my son's preschool class and he was the nonverbal kid who staff had to hold in their lap because he kept bolting for the front door. Now he's a quiet but polite and extremely talented kid.