As mentioned, Zenimax is more or less just a "holdings company." They themselves have produced nothing; all of the production and publishing is done by one of their subsidiaries. Bethesda Softworks was the first: later, gamesas was split to create Bethesda Game Studios as a distinct entity from Softworks, with BGS being 100% a developer and Softworks 100% a publisher. This was brought on, I believe, by the increasing number of titles that gamesas was publishing.
As time's gone on, the number of game developers has increased: Vir2L was created to handle the "non-flagship" platforms, specifically things like mobile devices, with their first major releases being the first three
Elder Scrolls Travels games. Later came Vir2L Online, which, I BELIEVE, is what was re-named as "Zenimax Online." (they either did that, or disolved Vir2L Online and created ZO at about the same time) Later has come a number of acquisitions: first iD, then Arkane, and along the way and still a number of lower-visibility ones, such as Tango and MachineGames, companies founded by developers that'd worked on
Final Fantasy,
Resident Evil,
Riddike, and other series.
Ever heard of Fujisankei Communications International? Well, they published Ultima 1 to 7
That's SLIGHTLY inaccurate: FCI (specifically, their subsidiary Pony Canon) was responsible for the console ports of the games. (and the Home PC versions that were Japan-only) The original PC versions were, indeed, published by OSI themselves. So anyone that played the original PC games would never have heard of FCI because FCI didn't touch those.