It's basically just a graphical update, retouching the textures, some shadows, some (not all) formerly pre-rendered cutscenes are now rendered live, some QoL features were added, and they re-introduced a formerly scrapped day-night cycle in SR1, making some areas practically unplayable without a torch, especially the cathedral at night. Thats it for the main game, no further changes has been made. Even some misplaced texture mappings have been "preserved". Even the out-of-bounds vampire model in SR2 is still there were she was in the original (even tough I kinda like they didn't removed her). For all I know, they used the same engine as the originals. You can switch inbetween modern and old graphics via F1 seamlessly.
They packed in some of the lost levels of the original SR1 concept under the "extra" section in the launcher for you to play, notably the undercity with the human priestess, the turelim territory with a boss fight, and Kains retreat, all dysfunctional and only meant for you to be explored. With the help of cheats, the smokestack is actually beatable, and crashes your game when you do. Also, some areas of them can't be reached without cheats.
Some of the translations are clearly just AI without any double-checking. That sometimes leads to some funny translations, like "Melchiah" being translated to "Milchtier" (milk animal) in German.
I appreciate the effort they went through, but the visuals were never the important part of the game in the first place. The lost levels are actually the highlight for me. They are not much, but seeing them gave me some closure. I do wonder why no attempt has been made to restore the original SR1 ending as an "extra", since they did seem to have complete access to the source code, and even audio translations for a lot of the dialogue into many languages pre-exist.