P.S. I tried setting a bunch of my other, smaller projects, to arm64, and they all worked without a hitch. So, it's not something wrong globally in my environment.

I'm running Version 13.1 of xCode. The project is already in GitHub, but it's huge (source tree is nearly 400 GB), and I'd prefer not to make it public (since there are people developing a commercial product based on some of its internals, and they have IP concerns). I guess what I need to do is strip out all the guts and make sure that the empty skeleton does the same thing. I did make a test program from scratch, and it behaved the way I would expect (easy to make an M1-only version). What I'm guessing is that I'm using some library that's not supported natively on the M1; it contains these: QTKit, Cocoa, OpenGL, AVFoundation, ApplicationServices, QuartzCore. I was expecting that there would be problems like the one I'm encountering, but I was also expecting that I'd get a clear error message, like "sorry, Native apps can't use OpenGL; you must switch to Metal." From what I read, I wasn't expecting it to be OpenGL though (since it said it was deprecated but should still work). I don't know enough about the other libraries to guess which one might be the culprit. Or it might be something really trivial (like me somehow telling xCode that my hardware is Intel-based, without realizing it).

When I set both project and target to arm64, the build options are for 'My Mac (unsupported architecture)' and 'Any Mac' (it doesn't say ‘Any Mac (Apple Silicon)’). And when I then try to build it, it says: The run destination My Mac is not valid for Running the scheme 'FileRender'. My Mac doesn't support any of MAMFileRender.app's architectures. You can set MAMFileRender.app's Architectures build settings to Standard Architectures to support My Mac. (BTW, thanks for your help, Graphics and Games Engineer)