You have to understand that music theory is not a set of rules to follow, its a set of ideas that sound good to western ears and therefore are very commonly found in most music.

Not every song, but much western music, especially pop music and nursary rhymes, will stick to the same major or minor scale of notes for the whole song. Going outside of this scale is quite normal too and changing the scale/key multiple times in a song is also quite common.

The point of learning music theory is to give you a toolbox so that you can both recognise patterns in music you are listening to, as well as give you some ideas of what sounds good when you compose or improvise.

This is quite similar to mathematics where in school we dogmatically teach students how to do arithmetic in the base-10 euclidean system, because having deep fluency in one system is more useful than having a little understanding of many systems.

I certainly understand that in any system (not just music), some rules can be broken at certain times for various reasons (safety, aesthetics, etc.). To combine what you're saying with what I was saying, I would like to see, at least in this case, a better explanation of what exceptions are commonly made to these rules (e.g. using non-standard chords) and when they are made (e.g. in music from region XYZ).

In fact, I'd argue that such explanations are critical even when you're learning any kind of theory since it tells you when the theory breaks down, whether that's because it simply doesn't fit the task, because people like to get creative, etc.

You've missed my first point. It's not rules. There are no rules to break. The "broken rules" simply describes systems within music that you haven't been introduced to yet. For example, major and minor are also called "ionean" and "aeolean" "modes" respectively, and these sit within a set of 8 modes. There are also blues and jazz scales, names for music that break out of standard tuning. There is a style called "12 tone" (of which an example is Also Sprach Zarathustra) which uses deliberate dissonance.

You simply aren't up to that part yet, because classical western music training teaches the more common ideas which are in music that the punters listen to first. (Mainly because people generally need a few years playing music before they develope a taste for these things anyway)

I am aware you didn't specifically ask for answer, but I was not satisfied with the answers you got :-), so I'll add my two cents.. I have 3 points to make. (1) The first is about 'when does it make sense for a song to use more than 7 notes?' WHEN we do this, we will often say "this song uses a key change". Some keys have partial overlap - note-sequences they share, and ranges where they differ. One elegant way to exploit this, is to let the song meander into the common range of the two keys, and then meander OUT of that range into a different key that we used to get IN to that range. This can produce a cool surprise effect, a bit like looking at those optical illusion pictures that you can look at two ways. A similar trick can be used with rhythms that overlap, instead of frequencies that overlap.

(2) Where does the "rule" of 7 come from, ie what shapes it: As you know, notes have harmonic friends that they resonate well with. So when you are picking a 'colour palette' of notes that go well together, you will of course often pick such 'friends'. However, the more notes you already have in your picked pool, the harder it gets to add another note, that will still mesh nicely with all those previous choices. Your remaining choices will be more and more likely to clash; in particular it will be more and more likely to be "close" to one of your existing choices. And close notes clash. So, on a 12-note scale, 7 is about the optimal number of tones you can pick without them clashing too much. It is just a convention however, so some stubborn individual might come up with an 8-note scale. Once you start with 8 notes, you would be tempted to employ extra "OK I have 8 notes, but I try to avoid playing THOSE TWO back to back"-rules.

Then again, I often hear my 10-year old loudly playing .. sounds(music?) from tiktok and its ilk, and as an old geezer, I am starting to think that some of our youngsters have given up on scales altogether..

I have no idea what my third point was, at this point.

The sentence you quoted is a decent simplification but you probably shouldn’t take it too literally. It’s not really that the melody uses exactly seven notes. It is that these seven notes form the harmonic context that the chords and melody sits in. Normally that also means that the notes in question will be the most common ones in both chords and melody, but you can certainly use other notes as well.

A key is really an “I know it when I hear it” thing. The notes used are just one of many clues working together.