Afternoon, Lizid38!
You've got a good question here about using your GoPro Hero8 as a webcam on your MacBook Pro. Allow us to provide some assistance.
If nothing happens when you connect a camera to your Mac would be a good place to begin.
It might help to turn the GoPro to video mode, then connect it to your Mac and open the Photo Booth app. Check in the Camera menu in the menu bar for the GoPro to select, if given the option.
If you can use other webcams without issues, then you may need to use the GoPro Webcam Beta for MacOS. There is an awesome video there that walks through that process.
In case that doesn't solve this for you, let us know if other webcams work properly when they are connected and if you've ever been able to use the GoPro Hero8 as a webcam.
Wishing you the best and hope to hear back if you need us!