I'd never heard of Alt combos to emit characters

That's quote common on non-US keyboard layouts. The right Alt key is labeled "Alt Gr" on German keyboards, and it is needed for a lot of characters:

That's the layout used by Windows, and printed on the keycaps. It is called "T1" in the DIN2137-1 2012-06. You can also find these mappings on Linux, which usually adds some extra mappings not printed on the keycaps. There is an extended German keyboard layout called "T2" that I have never seen in the wild, compatible to "T1", but with extra characters on any character key. MacOS is a different beast.

And yes, using the German layout can be annoying to use when writing code in languages derived from C, about half of the characters you need have to be typed using the "Alt Gr" key.

Many, but not all PC operating systems accept any Ctrl key + left Alt key as a substitute for the "Alt Gr" key. Windows 2000 and newer do, MS DOS up to at least 6.22 does not. Maybe this was added for some small keyboards lacking the "Alt Gr" key.

Things get interesting with other layouts. The Swiss keyboard layout (https://en.wikipedia.org/wiki/File:KB_Swiss.svg) comes in two variants, Swiss German and Swiss French. Both variants are identical except for the three keys responsible for ä, ö, ü and à, é, è. The Swiss German layout has äöü in the unshifted state and needs shift for àéè, on the Swiss French layout, it is the exact opposite (äöü need Shift, àéè don't). Uppercase ÄÖÜ and ÀÉÈ aren't available, neither is ß (not used in Swiss German).

See also the wikipedia pages German keyboard layout and Keyboard layout, and of course the exhaustive list of usages in AltGr key.

Alexander