Old TV repairmen could swap tubes and look for charred parts, but when transistors and I.C.’s took over, you basically swapped out whole boards.
Other than simple things, a lot of stuff is black box in nature.
As an example, I had a Breville oven quit on me. There was an inline fuse (looks like a resistor) Simple. But I had to take out some 30 screws, and then I couldn’t find the part inside the cramped the case. So, even dead simple can be hard. A tension switch in my drier is obviously at fault, but it’s in the back, which is not accessible, so the entire drum and whatevers need to be removed to access it. Computers (not all of them) and phones are designed to be unrepairable. There are sites (forgot the names) that show teardowns and offer special tools, but good luck finding info on anything smaller than an entire logic card.
There may be books on how to build and test PC’s, if that’s what you are looking for. Make Magazine is back, and you might look at sparkfun, lilypad and any number of Arduino and Raspberry Pi publications for ideas, but other than the educational value of these publications, (I learned a ton from reading Popular Electronics), nothing specific about repairs that I know of, mostly new projects.
The more specific your question, the more specific answers you’ll get. Good luck.
For example, I had a pair of Samsung LCD monitors that stopped working. I looked up how to repair them and found that they suffered from leaking capacitors. Some video showed how to identify the capacitors and how to desolder an solder replacements. I followed the instructions and got the monitors working again. Another example is I have a Delco car radio from the 90s with burnt out illumination bulbs. Again there are walkthroughs on repairing these.
Really old stuff, with vacuum tubes, can have a "hot chassis" with mains voltage on it, and/or 200-300 volts on the plates of the tubes, be careful. Tube testers can be had, and so can tubes, for the most part.
Newer stuff, with transistors, usually has dead electrolytic capacitors that need to be replaced, and sometimes there will be other issues, most of the time it's a matter of tracing signals through a system to see which stage is at fault.
When you get to the newer stuff, with surface mount components, you'll need to have schematics and block diagrams in order to even start to figure out what's going on. The really new stuff has transistors and other passive parts so small you'll have to have a microscope to work on them with. You'll need hot air reworking gear to repair that type of stuff.
But first.... as I said earlier, start with the basics... programmers don't have good intuition about parallel and series circuits, etc. You can get multimeters and oscilloscopes very cheap these days.
If you want to get into radio, investing and playing with an RTL-SDR dongle and GNU Radio is a great way to get a feel for how things work, and to learn about negative frequencies, and all things signal processing related.
When you have I and Q (Information and Quadrature of it), you can tell which way a signal is "spinning"... and there are many things you can't do if you don't have both parts of the signal.
If you have two antenna near each other, you can look at the phase and tell where the signal is coming from, in terms of which is closer to the source, by that rotation direction.
If you think that may be too much to begin with, you are not mistaken! Often recommended is Practical Electronics For Inventors to begin with.
You may also want to evaluate these: https://hackernewsbooks.com/search?q=electronics