These are quite complex questions
to answer. Maybe this will help you:
1) Some time ago I wrote an article on this topic: 3 STEPS How to determine / calculate number of PCB layers
2) Do you mean "How do we know that what type of components are placed on what side of PCB" or "How do we know that what type of signals are routed in which layers"?
This is a very big topic
- Usually, big and hot components on the top, small components on the bottom (but there are much more factors playing in the decision).
- Important signals inside the PCB, ideally routed between two solid ground planes.
3) Very simple answers:
- yes there may be crosstalk between tracks, try to play with Saturn PCB Toolkit
. It's a free software and you will find there Crosstalk calculator. In the same software you will also find impedance calculator.
- Matching impedance is very important. I am speaking about this subject a little bit in this video: High Speed PCB Design Rules (Lesson 4 of Advanced PCB Layout)
. For impedance requirements, I recommend you to read datasheet from your chip manufacturer or you can for example check this document (starts at page 182): COM Express Carrier Design Guide
I hope it helps.