Microsoft recently rolled out a brand new Redstone 5 build for Insiders in the Skip Ahead ring that added several new improvements to the Windows "Sets" feature that has been in testing for a few weeks. When Sets was first introduced, it was incredibly rudimentary, missing features and functions that made it pretty difficult to use.
With build 17639, Microsoft improved Sets a fair bit, so it's actually usable now. It now has the ability to move tabs between windows, stability improvements when switching between tabs, and better behavior between File Explorer and Sets itself.
It's still not perfect, which is to be expected because Redstone 5 is still several months away from being finalized. There are a few other changes in build 17639, including additional settings in the Settings app, Bluetooth battery percentage, and more. We've got it all in our video walkthrough, so check it out!