How to set my default browser

How does my system know which browser is the default? Or indeed any default something application?

$ cat ~/.config/mimeapps.list
[Default Applications]
text/html=userapp-Iceweasel-EZC28X.desktop
..

[Added Associations]
application/octet-stream=emacs24.desktop;
application/x-virtualbox-vmdk=userapp-virtualbox-LJU14X.desktop;
text/html=iceweasel.desktop;userapp-Iceweasel-EZC28X.desktop;
..

The .desktop files correspond to one of the files you have under .local/share/applications/, e.g. this one for Iceweasel:

$ ls ~/.local/share/applications/userapp-Iceweasel-EZC28X.desktop
~ /home 🏠 ~ talks 💬 ~ bash ~ craftsmanship ~ db ~ dongxi ~ emacs ~ escenic ~ iam ~ java ~ js ~ language ~ latex ~ ldap ~ life ~ linux ~ llm ~ mac-os-x ~ mt-foo ~ network ~ norsk ~ python ~ quotes ~ running ~ security ~ travel ~ unix ~ various ~ vcs ~ webdesign ~ windows ~ discoveries ~ cv 🧙 ~
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