Just like this topic:
https://developer.apple.com/forums/thread/713211
Except this also applies to the latest Xcode 15 beta 2.
The Attach to process feature seems to be broken, as it is unable to enumerate a process list and display it for me to choose from. It just says "Getting process list..." forever.
Edit: I seem to have found the problem. It is dependent on having a project currently open in Xcode. I am trying to debug a binary from someone else without their projects, just to peek at the Metal scene capture, and thus did not think to open a project first. Opening a project makes the process list fill out.