Mac -> Ginger GroundControl Caster (Freeware) https://gingeraudio.com/groundcontrol-caster/
Mac -> Loopback application by Amoeba (about 100€): https://rogueamoeba.com/loopback/
WIN -> Loopback application (Freeware) : https://loopback.io
WIN -> Voicemeter (Freeware) : https://www.vb-audio.com/Voicemeeter/index.htm