Don't be silly. We've already covered this. The engine uses Havok physics (same as Dark Souls 3, Wolfenstein, and Civ 6), and the renderer was completely redesigned (more than once) by Bethesda as a 64-bit DX11 implementation.
… and the sad truth is that Beth still uses the old Gamebryo foundation under its Creation engine.
Gamebryo was a good engine 100 years ago (one of the first "component-based" engines), but its main issue still remains: it cannot properly synchronize its game systems (render, physics, network, game logic, etc)
BTW, this is the problem with basically any game engines that are not Id Tech.
If you are interested in the technical background, here is a quick and dirty overview:
Syncing game systems together is much much much more harder technically, than rendering trillions of polygons at 600 fps.
With the renderer, you can do shortcuts, and gain performance.
With the synchronization, you must do parallel, asynchronous and thread-safe programming, which is a very complex thing to do. There are no shortcuts. Either you can do it right (=you are John Carmack), or not.
So, the easiest thing to do is to hardcode the engine for a fixed frame rate, and make the synchronization frequency locked as well. No threads, no async subsystems, nothing. Paycheck.
On the other hand, Id Tech's game systems are running totally async in parallel.
… and that's the reason why Id Tech-driven games are super smooth and hyper cool to play, despite lacking certain graphical bells and whistles.