The best 10,000ft view that doesn't just point you at the code itself is probably https://bevy-cheatbook.github.io/gpu/intro.html. If you need finer details, you'll need to read the code itself, or ask in the #rendering(-dev) channels on the Discord.
Again, I'm not happy with the state of public documentation for this, but as seen with this release, it's still heavily in flux. I don't think we've had a single release since 0.5 that hasn't made major architectural changes of some kind.
> And what's the equivalance of Unity's Frame Debugger? Since Bevy doesn't have a GUI editor yet, I guess the answer is just to use Render Doc?
Yep, just use RenderDoc right now. I don't think we'll have a good solution for this for a long while, since building one of those from the ground up is a huge undertaking.