How can I learn about repairing Apple devices or Mac computers?

I have been a long time Mac user so I am pretty familiar with Mac OS. But I know almost nothing about how my computer actually works. I sometimes think I would like to learn about the inner workings of my computer and maybe buy an old Mac and get it back into working condition. I saw a Youtube video of a guy who found an old G4 iMac (I had one) in the trash and he took it home, took it apart, cleaned it, repaired and upgraded it, and was able to install a much more recent version of MacOS on it and now he is using it as a regular computer. Can anyone recommend, perhaps, a book, like "repairing Mac computers for beginners" or anything like that? I have not seen one on Amazon. Is there an online course? A Youtube channel? A website?

I remember the older Macs used to actually have a little door on the back or bottom and you could easily buy more RAM and upgrade your computer by opening the door and changing the RAM. I know in the last 15 years they have moved more towards having the whole system on one chip so the computers are not as easily worked on or upgraded as they once were. But I have seen companies like that sells parts and even have videos about upgrading and repairing relatively modern Macs. So it must be possible. Any suggestions would be greatly appreciated.

[Edited by Moderator]