I did some more research on this and found that my Windows system has
AppUriHandlers registered to start the Windows Subsystem for Android (WSA) if any of the following hosts are targeted when opening a URL:
kindle-vella.amazon.com
read.amazon.com
WindowsSubsystemForAndroid.microsoft.com
www.amazon.ca
www.amazon.cn
www.amazon.co.jp
www.amazon.co.uk
www.amazon.com
www.amazon.com.au
www.amazon.com.br
www.amazon.de
www.amazon.es
www.amazon.fr
www.amazon.in
www.amazon.it
Because of that registration opening any of those using the Windows start command will launch WSA on my system, for example by entering "start http://www.amazon.com" at a command prompt.
Most of them do not do anything obvious except start WSA and then open the URL in my default browser (Chrome). However using "start http://kindle-vella.amazon.com" launches the Kindle app within WSA and shows a screen with information about
Kindle Vella stories.
I do not know whether Amazon or Microsoft is responsible for this registration. They appear to be working closely on WSA and Amazon is responsible for the appstore there.