No post, PC turns on, winds down, stays on.

What motherboard do you have?

Your motherboard can have beep codes or have LED lights that show the post stage.

It will go CPU- RAM- GPU or something similar to that. If the light is stuck on CPU then that is most likely the issue.

If your friend would be cool with it, try his CPU or another one if possible. If it is the same socket.

You said you bought a entire new motherboard, PSU and ram just for it?

Let’s be honest, whats the chances you have two broken sets of a PSU, RAM, or motherboard.

It’s most likely the CPU.

You should sell some of those parts or at least you have extra for another :)

But again, try what I said, and tell me what motherboard you are using.