~/.config/fontconfig/conf.d/01-emoji.conf

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
 <alias>
  <!-- Change the string in the family tag to whatever font -->
  <family>serif</family>
  <prefer>
   <family>Noto Serif</family>
  </prefer>
 </alias>
 <alias>
  <family>sans-serif</family>
  <prefer>
   <family>Noto Sans</family>
  </prefer>
 </alias>
 <alias>
  <family>sans</family>
  <prefer>
   <family>Noto Sans</family>
  </prefer>
 </alias>
 <alias>
  <family>monospace</family>
  <prefer>
   <family>Noto Mono</family>
  </prefer>
 </alias>
 <!-- This adds Emoji One to the font families sans, serif, sans-serif and monospace -->
 <match target="pattern">
  <test name="family">
   <string>monospace</string>
  </test>
  <edit name="family" mode="append">
   <string>Emoji One</string>
  </edit>
 </match>
 <match target="pattern">
  <test name="family">
   <string>sans</string>
  </test>
  <edit name="family" mode="append">
   <string>Emoji One</string>
  </edit>
 </match>
 <match target="pattern">
  <test name="family">
   <string>serif</string>
  </test>
  <edit name="family" mode="append">
   <string>Emoji One</string>
  </edit>
 </match>
 <!-- Discord loads the system's sans-serif font family, add Emoji One to it -->
 <match target="pattern">
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit name="family" mode="append">
   <string>Emoji One</string>
  </edit>
 </match>
 <!-- Add emoji generic family -->
 <alias binding="strong">
  <family>emoji</family>
  <default>
   <family>Emoji One</family>
  </default>
 </alias>
 <!-- Alias requests for the other emoji fonts -->
 <alias binding="strong">
  <family>Apple Color Emoji</family>
  <prefer>
   <family>Emoji One</family>
  </prefer>
  <default>
   <family>emoji</family>
  </default>
 </alias>
 <alias binding="strong">
  <family>Segoe UI Emoji</family>
  <prefer>
   <family>Emoji One</family>
  </prefer>
  <default>
   <family>emoji</family>
  </default>
 </alias>
 <!-- Run "fc-cache -fv" after saving the file and kill and restart whichever app (like discord) and enjoy emoji -->
 <dir>~/.fonts</dir>
</fontconfig>