Hi,
If you really want to know more about motherboards and you have the time to study, you can look up all the chips and that will give you more information. For example, you can look up the CPU chip and you can get the data sheet for that in many cases. You can then study how that CPU works and what all the pins do.
You can then look for the support chips and see how they work.
You'd have to do this anyway even if you had a motherboard schematic.
A better place to start however might be with a more simple system, like with a Z80 CPU. It's a lot simpler than the current CPU's you see today and the support chips are simpler too. Once you know how that works, you'll at least have an idea how motherboards work so you can understand more about the more advanced CPU's and interconnects.
Motherboards also have multi layers as you know so might be a little tricky to repair if you needed to unsolder something. If the plated through hole is damaged it could mean it would be extremely hard to fix.
You'll also have to study switching power supplies, in particular, buck converters, because the multi core CPU's require several down converters to run their power pins.