And they all use different connectors, just in connectors you will/would need a fortune, getting pig-tails from a scrapyard might or might not be possible.
You can make a somewhat decent engine simulator, but crank/cam position sensors are best done using real reluctor wheels strapped to dc/3 phase(rc engines are nice) motors, you will need gauge clusters, ignitions, keys, because immobilizers most likelly wont even let your ECU power on, deleting the immo is possible, but what if the problem with the ECU is in the immo?
You will also need a fortune in ecu reader/programmers(unless you resort to ebay tools that might or might not work, or worse, not even support your ecu).
But, you can make a living out of coding and setting immobilizers in botched cars, one guy was quoted 10k(and lots of new parts, an a month of waiting time) from Audi to re-code a pair of keys on a Q7, one guy did it in 3 hours, just be prepared to be in shaddy forums, google translating russian, and getting random tools from random people and install random software to use said tools.
It as its niche, but you must be interested in it to get started, and dont hope to make rivers of money.
Component level repair in consumer electronics, is a pain in the butt, cellphones are all crap, HATE to repair cellphones, bonus points for Apple crap.
Laptops are a bit better, but, almost all forums where you could get bios dumps and schematics are going the paid way, ranging from 10-20€ a month to annual subscriptions over 2k €, at least some indian forums are still ok, almost all of them are as expected russian, google translate is your friend.
There are a lot of easy repairs, because Windows 10 loves to fuck(its indeed the right word) with UEFI BIOS, so you get some laptops that are an easy BIOS flash and its done, some you chase some random smd mosfet that gave the ghost and now it wont detect AC power, to cracked ferrites that lead to dead usb ports.