My sinks, showers, dishwasher, etc all have a separate PVC sewer line and only the toilet still uses the original steel one. Both sewer lines, PVC and steel, exit the basement separately, but I don’t know if they stay separate all the way to the sewer connection at the street or combine outside. Thanks previous owner /s.
That said, I’ll have to check the vent configuration. I think there’s only one vent and want to say it’s shared between the two, but I have no idea. When I bought the house years ago, the inspector didn’t make any notes about that, so I assumed it was all good.
Thanks for that diagram, btw. Will see how mine compares and hope the plumber doesn’t just recommend ripping everything out and starting over xD