Phew where to start…

I like your post, but there are multiple questions and a lot to talk about, however I‘ll address only the main one: What’s a key?

When you hear one frequency, like 440 Hz, your brain will see this tone as the key. Add a different frequency (another white button on the piano) and go back to the 440 Hz, your brain will like it. What your brain also does is comparing ratios to whatever it hears. The simpler the ratio the more you‘ll like it. 2:1 or 3:2 are very nice.

I‘m not sure if the brain is prewired to anticipate tones with simple ratios or if it is a learned skill. Ultimately your brain likes it if you go back to the first tone or the simple ratios of it and everything in between is the skill of a songwriter to provide you with a nice journey.

Every simple ratio note derived from the initial frequency belongs to an extent to the key of this initial frequency. You will not like random frequencies mixed with the small ratio ones.

If you wait a few seconds after the initial note you can select a random frequency and this will then be ne next new key without bothering you. But play them together or in short succession and you won’t like it.

I‘ve jumped over a lot stuff here and it‘s hard to explain music theory without sound, so here is a video with a detailed and visual description of this topic: https://www.youtube.com/watch?v=mdEcLQ_RQPY&list=PL618khw0A-t5A7TyZmxAux_v_C3O7rO2l