Note: bitwize is talking about how to do it on Linux. Which is the best way in my biased opinion. Perhaps not the best mapping for people who use it regularly but is awesome for those who use it irregularly. We can usually guess how to do weird diacritics without having to look it up.
And when I can't think it up, I have a bookmark for my locale's upstream compose sequences file: https://github.com/X11Libre/mirror.fdo.libX11/blob/master/nl... and/or https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master...