With SRMs the power calculation is performed in the head unit. The SRMs (direct force power meters - ANt+ type 4) send Torque and Cadance. There is a calibration protocol that the head end can instigate when the rider is freewheeling but the cranks need to be unloaded and at quarter to nine for that to be repeatable and accurate. It is unclear (to me) if Zwift supports SRM auto-calibration - I’ll test it after my trip to the shops …