To get started is to read something unpretentious on circuitry. To be able to distinguish the capacitor from the resistor and understand what they are for (of course, it all depends on how deeply you want to "go down").
Then read by elementary logic (elements and / or / not and the theory - Boolean functions, deMorgan rules, etc.)
And you can take, for example, this book - [ https://habrahabr.ru/post/259505/ (a textbook by David Harris and Sarah Harris “Digital circuit design and computer architecture”).
It will be more clear further.
As a bonus, I would recommend to find a selection of Radio magazines and there to find a description of the computer Radio-86PK . The “computer” itself and the components are very outdated, but I think it is quite possible to shift it onto modern rails. They even have their own operating systems for it.
But with porting Linux - this is quite a difficult task. If the processor is standard, then you need to look for a compiler for it (or maybe already have a ready-made assembly, which will need to be added with the right drivers). If not, you will first have to make a version of gcc or another C compiler for the target platform.