In Spanish, we consider Ñ to be a different letter, with its key in keyboards and under a different chapter in dictionaries. Of course you could say that it's just an N with a ~. But that tilde is not defined for any other letter. Why did they do it? In medieval Spanish it was common to write two Ns in many words, with a different sound. For example: "anno", which is now: "año" (year). Two Ns was very similar to M so the tilde was added to remark that it was two Ns, not one M. Later, they just wrote one N with the tilde.
Exactly and ä is never just a, ö never just o and ü never just u. Words like "Uber" just feel wrong from a German perspective.
Also, ß is ss and since words don't start with Ss there is no need for an uppercase ß. When whole words are capitalized ß turns into SS.
Of course there is a little edge case where capitalization isn't reversible. For example, is the capitalized version of the name
MASSMANN to be converted to Maßmann or Massmann?
That is in my opinion the only reason the uppercase ß was added to Unicode. To resolve this ambiguity. It has no place in proper German typography.
Worse, in Spanish it used to be that 'll' and 'ch' were until recently digraphs considered separate letters, which meant that they would sort as such too. That is incredibly annoying to implement.
Technically the ij is a digraph and sometimes a ligature. Although computer keyboards have never really supported that but some mechanical keyboards for typewriters used to. Most modern Dutch people would simply be typing an I and a J and wouldn't necessarily know that they need to also capitalize the J at the beginning of a sentence (Ij would be incorrect). Not a lot of words start with that but some place names do.
The Dutch keyboard layout is effectively US international. There are no special characters that that keyboard supports; including all the accents/modifiers we inherited from French, German, etc. Spelling without those is now correct.
This is something that has evolved over the last decades. In the eighties, people would memorize the character codes to produce letters with those. I remember having a card with the right key combinations for word perfect. This is not a thing anymore. People just skip it and the grammar and spelling rules were actually modified to not require these anymore in most situations.
Funnily enough, the Y, is not commonly used in Dutch and usually referred to as the Greek IJ but pronounced the same way.
For me the Unix/Linux compose key and compose key sequences is where it's at. It's much nicer than the US international keyboard input method, though if I were constantly typing in a language that requires [lots of] accents/diacritics I might have to switch.
And, while its not the case any more, but illustrates how arbitrary the "new letter" vs. "new sound for old letter or combination" is "ll" also was considered its own letter for ~250 years.
