We can do this by creating a .desktop file. I named mine "Choosy" just to keep things consistent, but feel free to use whatever name. To the desktop environment this will be our "browser" but in reality it will simply invoke a script that we create.

[Desktop Entry]
Name=Choosy
GenericName=Browser Chooser
Comment=Choose what browser to use for specific links
Categories=Network;
Exec=choosy %u
Type=Application
MimeType=x-scheme-handler/unknown;x-scheme-handler/about;text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;

The important bits are the `Exec` and `MimeType` properties. With `Exec` we control what to execute when a link is clicked, and with `MimeType` we let the desktop environment (Gnome, KDE or others) know that this application of ours should be available as an alternative browser choice.

Create this desktop file in ~/.local/share/applications/, log out and log back in and you should be able to find it listed as an option for default browser. For example in Gnome it will be listed under Settings - Details - Default Applications - Web.

I keep two sets of URLs for different matching types. I'm interested in matching both exact URLs like https://www.mozilla.org and also partial URLs such as drive.google.com or app.gotomeeting.com.

Create two files ~/.config/url-start.txt and ~/.config/url-is.txt

url-start.txt matches partial URLs, not considering the protocol, URI or parameters.

drive.google.com
docs.google.com
meet.google.com
app.gotomeeting.com
hangouts.google.com

url-is.txt matches exact URLs. If the protocol. URI or parameters change it won't match.

https://www.mozilla.org
https://en.wikipedia.org/wiki/English_Wikipedia
https://blog.cesarolea.com/posts/emacs-native-compile/

To tie it all together is a bash script. The script receives the URL and compares it with the values found in the lists above. Depending on where it's found one browser or the other is selected.

#!/bin/bash

URL=$1
DOMAIN=$(echo "$URL" | awk -F[/:] '{print $4}')

## Domain starts with
if [[ $DOMAIN =~ $(echo ^\($(paste -sd'|' /home/your-user/.config/url-start.txt)\)$) ]]; then
    chromium-browser "$URL" & disown
elif [[ $DOMAIN =~ $(echo ^\($(paste -sd'|' /home/your-user/.config/url-is.txt)\)$) ]]; then
    firefox "$URL" & disown
else
    firefox "$URL" & disown
fi

The logic itself is very simple, the most complex part is parsing the files. It's also very easy to extend: say you want another list of URLs, ones that should always open in another browser such as Nyxt. Simply create another list, name it url-nyxt.txt and add another elif block. Each block is evaluated in order, so if the URL is found in other files it will be matched by the block that is found first.