The unrar cli is published under a source-available license that you can't use it to build a rar compressor. So using knowledge from reading that might be fair game for building a rar decompressor, especially if you follow the usual clean-room protocols (one person reads the code and writes documentation, the other reads the documentation and writes new code).

Not sure how that will affect winrar in the long term. Of course people now aren't forced to download software to open rar files. But it also makes it much more viable to send .rar files to random people. And actual Winrar still offers you a lot of features that Windows doesn't have and seems to be a lot faster, so there are still plenty of reasons for power users to use it.

Personally I own a winrar license primarily for its features when creating archives, and Windows isn't encroaching on that.

I don't know but this is their response to the Microsoft's announcement.

https://twitter.com/WinRAR_RARLAB/status/1663493174642323457