How can I use continuity camera with my Mac studio when it isn't connected to a webcam to begin with?

I suppose that the issue is to get your mac to acknowledge that your iPhone is available as a camera.

See if any of the information on this page can help:

Continuity Camera: Use iPhone as a webcam for Mac - Apple Support

In particular, try to see if you can access your iPhone as a camera in an application like FaceTime.

I expect that once it is recognized, it will also appear in Zoom.

Please do let us know of your progress on this.

I also have a Mac Studio, but alas my iPhone is too old, so I can't test this; I hope to get a new(er) iPhone later this year, and this would be a great thing to have.

Or perhaps if you could borrow a cheap webcam and attach it to the Mac Studio it might unblock your access in Zoom and then let you use the iPhone.

Would suggest exploring the information contained in the Link at the Top Left of this page.

Incase it was overlooked and it does occur

Here is the link below

Related Article: Continuity Camera: Use iPhone as a webcam for Mac

Off topic, to an extent that OPs question, not receiving Auto Notifications from Apple of new posting ?

I posted above at 05:41 today and yours at 06:17 today