You need a capture card. If you want to use it with OBS on MacOS, you need a capture device similar to:
BlackMagic (Intensity Shuttle)
AJA UTAP HDMI
Magewell USB
Generally avoid Elgato due to nonexistent Mac drivers, although some have said their CamLink product will work with OBS on MacOS. Under no circumstances get an HD60 or any variant thereof; they will NOT work with OBS under MacOS.