And they are also supported by kagi.
It's nice that kagi lets you define your own, so I can have custom ones across browsers / mobile / desktop... So long as I'm logged in and have configured kagi to be my default search engine (my phone defaults to ddg and sometimes I might use a browser in a VM or something).
Wanna check if Thunderbird v115 is in arch repos? Ctrl + L, !archpkg thunderbird
Boom!
My favourites:
!w <term> searches <term> inside Wikipedia
!so <term> searches for that term inside stackoverflow
!a <term> searches inside amazon.com
!ai <term> searches inside amazon.in
!arch <term> searches inside arch wiki article for that term
!archpkg <term> directly searches for archlinux.org/packages
Also, I just learned that there is a "!hackernews"