So after giving this a try, I get a missing character in the mode line:

Any suggestions?
Answering my own question: (after finding https://github.com/hlissner/.emacs.d/tree/master/modules/ui/doom-modeline)
brew tap caskroom/fonts
brew cask install font-fira-{sans,mono} font-dejavu-sans
and
M-x all-the-icons-install-fonts
are needed.
All of the closed duplicates seem to have been solved by M-x all-the-icons-install-fonts, but this hasn't resolved the issue for me. I'm on Ubuntu 18.04/Emacs 26.3 with a new Doom installation.
M-x all-the-icons-install-fonts seems to execute without any errors, returning two messages :
fonts downloaded, updating font cache, and Successfully installed all-the-icons fonts to /home/<user>/.local/share/fonts. I do indeed see all-the-icons.tff where the output said it would be, but after restarting Doom, the icons in the modeline are still broken. I also tried doom sync and doom refresh after the fact.
@ammkrn
I also tried doom sync and doom refresh after the fact.
These two commands are aliases. No need to use them both.
but this hasn't resolved the issue for me
The two common causes for this are:
fc-cache -f -v manually. Check fc-list for the all-the-icons font.In any case, you'll find the fonts here: https://github.com/domtronn/all-the-icons.el/tree/master/fonts
Try downloading and installing them manually. It is available to Arch Linux users via the ttf-all-the-icons package, and NixOS users via the pkgs.emacs-all-the-icons-fonts package as well. I'm not sure if it's packaged for Ubuntu.
Thanks for the quick response. FWIW the only one that didn't download correctly was material-design-icons.tff which had a size of 0. The other five have the same sha256sum as the files from the repo.
For anyone else that runs into this on Ubuntu, the steps to fix were :
all-the-icons/fonts) from wherever Doom downloaded them to, probably .local/share/fonts. .ttf files from all-the-icons.el/fonts into .local/share/fonts
Most helpful comment
Answering my own question: (after finding https://github.com/hlissner/.emacs.d/tree/master/modules/ui/doom-modeline)
and
are needed.