Long story short, I'm a long time programmer, software engineer by profession. I have that part figured out. However, I'm new to the hardware side, especially designing/building electronic circuits. I have a couple of questions.
First, I know it's possible to have an external power source for a breadboard. I've seen a few topics here on the forum. I see that there should always be a common GND, and I think that makes sense. What I'm still unsure of (due to being noob at this stuff) is how to prevent the external power from making its way back to the Pi. Is it correct that something like a transistor is good for this? I've also seen other components mentioned which shield the Pi from the external voltage.
Then my second question is, with an external power source for, say, a group of LEDs, how do I switch them off/on via GPIO (the Pi) while keeping the Pi's current at a reasonable level?
Maybe this is a lot to ask. Maybe I'm not being specific enough. Do let me know if something is unclear, I'll try to clarify. Not looking for anyone to do the work for me, hence looking for resources. Schematics that demonstrate these techniques, noob resources I can look at to understand how these things work, etc.
I've been on Google and YouTube a lot lately. It's just kind of overwhelming and a lot of the stuff I've found assumes a moderate knowledge of electronic circuits to begin with.