There are various graphics and gameplay tweeks but the only significant change from original to SE in terms of engine was going from 32 bit to 64 bit and DX11. This meant the game could use more RAM, use higher resolution textures, load more objects without crashing... things like that.
This is what they have been doing for years. They have constantly added to it, made tweeks and changes but the actual engine, the base framework is the same one they have been using since the development of The Elder Scrolls 3: Morrowwind in the late 90's.
I've heard they are STILL using it for Starfeild and probably Elder Scrolls 6 as well. Skyrim SE should have been that last gane to use this fossil, it's limitations were truly laid bare with Fallout 4 and 76.