He said he had trouble finding these online but there's a huge list right here:

https://adele.uxpin.com/

And here

https://designsystemsrepo.com/design-systems/

It's more visual design focused, but if you're looking for something more detailed, I highly recommend: https://refactoringui.com/

This is what I do for a living so if you'd like some more specific info, I'm happy to point you in the right direction.