`Meta + .` opens an emoji picker on KDE. And you can type unicode with `Control + Shift + u`.

I've been meaning to look into this, but what I'd really love is a composer which can insert any text, emoji, or unicode character based on whatever alias I give it. I'd probably leverage more of unicode if I should hit a shortcut and type wd to summon a wave dash (〜).

Windows does a similar thing with win+. (I forget if win maps to the meta key on linuxes)

~/.XCompose might be what you are looking for.

Support varies - GTK needs an environment variable to use it and Qt (since version 5) only uses the first code point of the result. No idea about Wayland support or alternatives.