Default English keyboards in Windows and Linux are awful for typing in English (which includes needing easy access to some accented characters, among other things). I accidentally learned to type on the International layout so I’m very familiar with it—unless you often need to make a very wide range of accented characters, wider than you’d need for French and Spanish, it’s a worse layout than the default macOS English keyboard because it gets in the way when you don’t need those. AltGr is like the macOS default, but entirely much worse. Everyone, please copy them, they’re the only OS with a keyboard layout good for typing in English, which, it’s 2020 so that’s a big WTF.

[EDIT] examples, option = alt except remember it's positioned a little differently on macOS

option+c ç

option+s ß (german too!)

option+\ «

option+shift+\ »

option+e, deadkey for ´ which you can apply kinda like in US International

option+u, same for ¨ (I'm hitting space after to type the decoration on its own)

option+`, same for `

option+shift+/ for ¿ (or option+? if you prefer to think of it that way)

And that's just the stuff directly relevant to typing in Spanish and French (and German). Most of which are also relevant to fluent typing in English because we have lots of loan words and phrases from those languages, and quoting from them is also common. Then there's punctuation, currency symbols, and so on. All are better than any alternative I know of, let alone any other default keyboard, and if you never press option while typing you'd never even know they're there (that is, they never get in the way otherwise)

When I bought my first Mac in 2012, I was amazed to learn that the tricks I learned in 8th grade French class still worked nearly 20 years later.

Kinda, but you do need those to type fluently in English, too, which isn't an uncommon use case for a computer sold in an English-speaking country. We have tons of accented loan words from French and Spanish, among other European languages. We also need a lot more punctuation and "special characters" than any other default English layout I know of makes available.

It is entirely a joke that Apple's got the only OS, in the year 2020, that does a good job at the task of composing text in English, out of the box and without compromising other use cases. I don't even know of alternate layouts that are anywhere near as good on another OS. What the actual hell is up with that?

But what's the significant advantage of Mac OS's layout vs e.g. Microsoft Windows US International layout?

I found that using US-international mostly removes the need to look for the dedicated accented letter.

It works by making you compose accented character instead of typing them directly.

For instance, you have to press the grave accent key [`] then [a] to make "à". The apostrophe doubles as the acute accent and cedilla key, so ['] then [e] makes "é", [`] and [c] makes "ç".

It works for most diacritics in European languages (`, ~, ^, ', "). Also, to get the original character, press space (or double press them to double type them).

It's probably a tiny bit slower than having dedicated keys, but I find the consistency nice to have.

If you don't like this behavior then then I can recommend using a modified US-International layout where those keys are only dead if AltGr is held down [1]. Alternatively, you can use a version where AltGr acts as the modifier [2].

The latter is what I use since I only need it for Swedish special characters (åäö) which I get by typing AltGr+W, AltGr+Q, and AltGr+P.

[1] https://github.com/thomasfaingnaert/win-us-intl-altgr

[2] https://github.com/umanovskis/win-kbd-usint-nodead

Speaking from experience, this is very hard, I have the feeling it affects negatively your speed in both layouts, with AZERTY I was somewhere in between 90-100WPM, I decided to switch to QWERTY for several years and I'm consistently between 76 and 85WPM, now trying AZERTY back after using exclusively QWERTY I'm scoring between 60WPM and 70WPM.