With a new machine I think I'm getting serious about a move from C4D to Blender!

I'm sure I'll have some ridiculously stupid questions. LOL

First impressions... Cycles is amazingly fast! I found the Optix setting, and the Optix denoiser and the Adaptive sampling!!!

In a multi camera scene, how do I change which camera renders?

Hello prixat Welcome to the forum and no question will ever be stupid. To answer your question click on the camera you want to render from then click CTRL = Numpad 0 now when you hit F12 the rendering will be from that camera. I found this also that you might find useful in general since you are transitioning from C4D it is actually for anyone transition from any other 3D software

10 Tips for Switching to Blender

Yes, if you have an RTX graphics card, the Optix driver in the settings preferences is what you should be using. Latest versions allow you to incorporate the CPU to help with the rendering as well, even in GPU mode. I would also recommend trying out the Intel Denoiser node in the composite tab. I've actually found this to be even faster than Optix denoising on certain scenes. Of course, it will depend on your grahics card. I have an RTX 2070 Super. If you have a 2080 or 30x series, then Optix denoising might be better.

What's the shortcut for the pie menu, I want to turn viewport cycles rendering on and off? (Blender 2.92.0)

Pressing Z on the keyboard will bring up the pie menu for viewport views. You can toggle between wireframe, rendered, shaded, etc.

thanks

This page has a relatively recent (as of 2020) list of common Blender hotkey commands that might be useful. Most of them still work even in 3.0.

https://www.republicworld.com/technology-news/apps/blender-shortcut-keys-list-of-all-the-blender-commands-that-come-handy.html