"Is it simply another type of code vault for updates similar to github and other code hosting websites? "
No. When you sign-in to Windows Insider Program and link your PC to program you can get different level of Cumulative Update previews or full Windows development build previews for testing. What kind of previews you get depend on which preview channel you select.
You can start to read from this page: https://docs.microsoft.com/en-us/windows-insider/
Note: It's not recommend to use previews as main OS on your only PC. Most of insiders use spare PC's, dual booting or virtual machines.