16226 is being discussed, here: https://forums.mydigitallife.net/threads/discussion-windows-10-insider-preview-build-16226-pc.74569/
Generally speaking this is because people who are really in the know about the internal happenings, what's gonna go out when etc... don't talk about it.
I am trying to look for windows 10 build 16215 (x64) but I cannot find it anywhere. UUP, archive.org. etc. Can someone please share me this build because it is so hard to find online.
There is no official ISO of this build, so only UUP would offer you a trustable ISO. If you would get it outside the UUP project who would be able to tell you it's a trustworthy ISO, not containing some extra's you don't want to be present?