How did migration from PoW to PoS happen mechanically?

Hey all. Mined Eth a bit in college. Digging into crypto a little deeper now a little later in life. I've read a lot about the pros and cons of PoW vs PoS and I'm not looking to start a discussion on that here.

What I'm curious about is mechanically HOW that migration happened (or will happen). I found a report from a few years back that estimated 25,000 full eth nodes, I imagine there are a considerable number of miners outside of the major pools (may be wrong about that?). Can someone explain or link an article as to how the whole network of nodes and miners got updated to a new work model? Is Eth 2.0 going to be a separate currency? What happened/will happen to miners/nodes that failed to upgrade? Who decided to do the upgrade? Is it possible that, if enough of the community decided didn't "buy in" to the change of model, the movement from PoW to PoS wouldn't have happened?

Thanks in advance!