Difference between scale and key?

Originally, I thought that key and scale were essentially the same thing. I thought "key" was just a way of saying that (for the most part) we will be using notes from a certain scale. Maybe, C major, maybe B minor etc… But, the more I’ve studied theory there seems to be a deeper, more fundamental difference between the two.
For example, apparently you can use multiple scales within one key. I don’t understand why the key of C major would be called C major if you can use notes from other scales… My first thought would be that multiple scales can use the same notes, which makes sense. But, then my next question would be, can you use notes outside of the C major scale in the key of c major?
I've also heard “it all has to do with the concept of tonality”. I have a vague understanding of tonality, so how specifically does tonality determine what can be played or should be played within a certain key?
I feel like if I've learned anything in music theory it's that there are a lot of rules, but even more exceptions to the rules. Just saying that so you know I get that your answer probably won’t be as cut and dry as I'm asking haha


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A scale is ALL the notes in a key

A song could be in the key of say F# but it doesn't mean it will use every note in that key.

The main reason keys exist is because notation was written by hand, ink was a premium and if a piece of music had a bunch of sharps in it then it was easier to just note it at the beginning of the piece. Now over the years a BUNCH of amazing work has been done to tie all this together and deal with tonal centers as you mentioned earlier, keys change the tone of a piece quite a lot.

And if it makes you feel better I have studied this shit for decades, have a masters degree from Berklee college of music am still studying with a composition PHD that teachers music and to this day feel like I am hardly scratching the surface of this stuff. Its all pretty remarkable.

Here is a great video that should help you get started

Thanks! I didn't know what I was getting into when I started learning music theory haha there are a million more questions for every question you feel like you've answered lol