"I want to change my car engine from V8 to V6.

Plan on putting a sign in the window, "For Sale As Is". That's how most of these projects end up. Then go out and buy what you want. Even if the new engine bolts to the transmission, the engine mounts will be different so you'll be changing the cross member. Also different will be the radiator, front springs, Engine Computer and under-hood wiring harness, instrument cluster, Body Computer, power steering and AC hoses, and exhaust pipes.

If you want to know about general electronic theory, find some used text books at your nearest community college library. I have a lot of books too to get rid of. If you want to know the specifics of your vehicle, order a service manual through the dealer.