I think they're basically the same. Both use the work profile, freezes and un-freezes the apps when being and not being used etc. If i remember correctly, the most notable differences are that it's possible to block contact acces in Shelter but not in Island and Shelter is FOSS while Island is not (but the fork 'Insular' is.

https://f-droid.org/packages/net.typeblog.shelter/

Shelter is an app that takes advantage of the work profile in android to install apps in that profile and makes shortcuts for the app in the normal profile. So it feels like you're just using an app as usual but the app is pretty much sandboxed away from all your info.