Okay, hasty post:
Quick experiment you could try: Take your phone and call someone in your immediate vincinity and let them answer. Ask for their phone and push it close to yours, mic to speaker and move them around. No guitar needed as the speaker/mic membrane oscillate in place of a guitar string.
Feedback can be manipulated in many ways, with distance, direct interference with the oscillator(finger on string, volume-control on an oscillator in a synth etc.), air moisture, temprature, amplitude, equipment etc.
A feedback loop can just as well stay as an electronic signal, being fed back in before it goes out through a speaker. This, called an internal feedback loop, or no-input mixing, and is much easier to isolate from the other sounds in a chain, making it possible to limit and mix like any other sound signal. Many of the same parameters, like physical distance(as in length of cable, or how far the signal has to travel), affect an internal loop as well as an external one. External and internal loops can also be combined for further exploration. If you have a stereo guitar pedal, you can try running it in mono, with the guitar going into the right input, going out of the right, then going into left in and then out again. Or you can cross the left/right
An internal feedback loop in a mixing console can act much in the same way as an oscillator in a synthesizer can. Many oscillator circuits applicable for both sound and/or control is based on or incorporates positive feedback.
Just as an example of what can be done with feedback: I recently amplified an audio-signal with several positive feedback-loops running through a series og vegetable batteries powering a low power schmitt trigger oscillator circuit, to a degree where you can get a shock by touching the signal path.
I’m sorry if I’m making myself difficult to understand, and I’ve probably made some errors, but I hope this helps a bit.