I would suggest to read the intel documentation about their processors. There are a bunch of books some dealing with system programming because this is what you are asking here. From the history of computers and their individual operating systems there are a lot of solutions for handling multiple execution pathes. Some used segmented memory management schemes noadays you have paged memory management in most places. Because you have so many questions how things work in detail that I even suggest to learn how the old things worked. They started with more simple architectures and started to extend them step by step. This way your understanding grows the same way.
I have an older documentation available but also some very old books about intel i486 and Motorola 68000 CPUs. These are old but so trivial that they are easy to understand.