Microsoft does not have that. Avalonia UI is based on Microsoft technology but it's a different company. Basically they are a competitor to .NET MAUI but there is also some collaboration between these two projects, which makes sense as they follow a different paradigm.
The .NET Foundation is something that definitely has close ties with Microsoft, but technically isn't Microsoft. .NET Foundation does a lot of things, but one of them is make sure that open-source projects keep being maintained.
A lot of open-source projects rely on people from the community that do work in their free time. Sometimes things happen and they don't have any time to work on it anymore. Instead of the project going to waste, the .NET Foundation is there to make sure it can be continued, among other things.
So all projects that fall under the .NET Foundation can very much be competitors and all sort of projects, but it's not necessarily developed by Microsoft. And of course we're all friends after all, it's just tech <3
Hope this clears it up a bit.