Looks like
HexOS -> TrueNAS Scale -> Debian
At the 12:46 mark in the LTT video they made covering the beta you can see the login page for TrueNAS Scale and Scale is based on Debian, according to the release file on my TrueNAS Scale install.
I don't know how to make the embed not full screen, but the link is timestampped so it'll jump you straight to 12:46.
And my own scale install's release file:
Though the versioning on the TrueNAS site aligns more with how Ubuntu is numbered, but Ubuntu is based on Debain anyway so there shouldn't be a huge difference.
https://www.truenas.com/truenas-scale/
