It's not like the teacher is ignoring the existence of people in their classroom. They join the zoom so they can hear the discussions and the answers kids at home are giving...
I've just finished up my 3rd week of co-seated classrooms. All 7 of my classes throughout the day have kids face-to-face on campus and kids at home. I still look at and talk to the kids physically in the room exactly the same as I always have, I also just look into the camera and speak to the kids at home, too. I call on kids in the room to answer questions, and I call on kids at home to answer questions.
If I want to stick kids in small groups I can do that with Zoom. Zoom can split them all off into little groups and then automatically bring them all back together after whatever time limit I set. The kids physically on campus can just join the Zoom call so they can be part of small group discussions with the home kids.
It's really not that hard.
I share my projector screen on the Zoom call, so what is being displayed on the wall by the document camera is also being streamed to the kids at home. So everyone in the class is still seeing the same thing, it's just kids in the room look at the wall, kids at home look at a screen.
Everything is going pretty well and the kids who are coming to campus are enjoying being around other kids regularly again, even with all the extra procedures. They wear masks all the time except lunch. Lunch is very spread out. I have to spray a disinfectant/viricide on each desk a kid sat at between class periods, we minimize the use of shared resources and keep everything digital as much as possible.