The motherboard pcb has 6 layers, which a fair few hobbyist fabs are capable of doing these days. Everything else is just two.
I mostly went with OSHPark because their capabilities seemed top tier, and a couple of friends had good experiences. JLCPcb looks like it would’ve been cheaper, but I’m not so sure about them.
Certainly I’m already having a good time with the quality of the OSHPark boards. They can take an awful lot of reflow work and crappy soldering and still be perfectly fine! By far they’re the most durable PCBs I’ve worked with.