openSUSE is an odd mix because they have a very good relationship with SUSE and Tumbleweed and Leap have different hierarchies. As a result, openSUSE is both upstream, apart from, alongside, and a derivative of the corporate distro.
openSUSE Factory is where development happens that eventually becomes openSUSE and SUSE Enterprise Linux (snapshots of Factory make up Tumbleweed). SUSE stabilizes a core system for their corporate customers and shares those binaries (as of 15.3) and source with openSUSE for Leap. openSUSE maintains a larger number of backports packages that are shared with SUSE as as community supported software repo.