There's something a bit upsetting about how finding it online is faster and easier than using an application purpose-built for this purpose (Character Map)
For real though, Linux Mint comes with what seems to be a clone of it, name included, and I'm pretty sure I've seen other clones of it integrated into writing software. There have been plenty of opportunities to improve on the formula, and the experience is improved slightly, it's just not enough.
I admit I've never used it, but it seems to require you to know in advance the key presses to get the character you want, so it's not going to help if it's a character you only use rarely.
I actually find it a lot easier on mobile, because you can see all the symbols available to type without having to memorise them or have 2-4 different characters printed on each key. Gboard has almost every special character I ever need to use accessible in its two extra screens, and accented letters like êëéèē accessible by long-pressing the base letter.
Unexpected Keyboard (on F-Droid) is also fantastic for extra characters, give it a try, but I don't use it as a daily driver because of lack of spellcheck and glide typing.
Samsung Keyboard literally lets you design your own keyboard layout in a surprisingly robust and rich way. I don't know if it's available on non-Samsung phones though, and I can't wholeheartedly recommend it because it has a bunch of flaws and quirks. For example, every once in a while it seems to do select all + copy + paste, without you going anything besides typing normally. This can scroll the text to an inconvenient place, and remove special formatting. On YouTube if you're replying to a comment it destroys the username you're replying to, replacing the special highlight with just their name in plain text.
If you got compose key (linux, mac, windows with third party software), then those are trivial:
ë ñ ũ ü, and even åâăāãȧaąàáæª₂2²
Goes like Compose e ", Compose n ~, etc
But a thing to note that resulting letters are generic and not region-specific,
like that ë (U+00EB LATIN SMALL LETTER A WITH DIAERESIS)
is not the same as ё (U+0451 CYRILLIC SMALL LETTER IO)
Which might trigger spellcheckers or not even be displayed in certain fonts
There's also apparently some weird combos like Compose+:) for ☺ and Compose+CCCP for ☭, but no easily available keys for greek letters unless you tweak configs...