Honestly, I mostly gave up on it. It’s one of the main reasons why I wrote xiate: It uses VTE and thus GTK/pango, which does all the nasty magic. Emojis and other fancy stuff displays just fine in my terminals. (Needless to say, I spend most of my time in terminals, so that matters the most.)

My WM, though, uses simple Xft and that means I’d have to implement font fallback myself (see various suckless tools, they do it). I’m too lazy for that. The code without font fallback is easy to read – *with* fallback, it’s a spaghetti mess. If I needed font fallback, I’d probably sacrifice a goat and just use pango. There actually is a “pango” branch in my private repo of my WM, but I hardly ever need font fallback in window titles, so it’s just not worth it.

I have noto-fonts-emoji and ttf-unifont installed, which covers a lot of things. Never needed to fiddle with fontconfig (in that regard).

The glyphs (?) I need the most are part of my main font (https://uninformativ.de/git/qsqs). It’s not a lot. The rest comes from uw-ttyp0 (https://people.mpi-inf.mpg.de/~uwe/misc/uw-ttyp0/) and univga (http://www.inp.nsk.su/~bolkhov/files/fonts/univga/). And then probably fallback to DejaVu Sans Mono.

tl;dr: GTK/pango is fine due to witchcraft, everything else is a mess. :-)

Wasn’t there a command line utility of fontconfig that could show you which font is used to display a certain character? I don’t remember anymore. Would be interesting.