A few questions about Passive Optical Networks.

I don’t currently work in IT. I’m a computer science student and I’ve always been fascinated by how the internet works, and I think I’ve got most of the basics down with how passive optical networks work, but I have a few questions. I previously posted in another subreddit, but they said that is one would be a better fit. Please excuse my ignorance.

I want to know more about upstream bandwidth allocation. What protocol(s) does the OLT use to tell the ONTs when to transmit upstream data? I suppose that each PON variation is different, so for this purpose, I’d like to know the protocols for DWDM-PON and GEPON. I think I’ve read somewhere that it’s got a gate, request, and acknowledge command, but I don’t know the name of it. Is it at OSI layer 3?

I live in a rural area and I doubt I’ll be starting an ISP, but would it be theoretically possible to split a fiber at each service drop instead of having the splitter at the central office or “stacked” splitters (please forgive me, the name of that configuration also escapes me)? Would that be horribly inefficient? Could an asymmetric splitter do such a thing and still have the network go 20+ km?

Why do optical amplifiers cost so much? What’s the best network configuration so that you don’t have to run so many miles of fiber (tree, ring, any others I don’t know about)?

And finally, is it possible to stack a passive optical network on top of a passive optical network? More precisely, is it possible/easy to configure (I assume probably not feasible) to have each ONU on one passive optical network feed into an OLT for a smaller passive optical network?

I apologize if these questions are too hypothetical, if these are basic questions, or if they’ve already been answered. I welcome all answers. Thank you.