I am not gaming engine expert, but my observation so far was, that there are four major engines in use by creators.
RenPy (pretty much PowerPoint with some click flow logic)
Unity (seem real-time 3D capable engine, Twist is good example of its capabilities)
RPGM (outdated right off the shelf, looks mediocre and even with great story and inlay image renders, can’t attract much attention of nowadays players. The only creator who executes RPGM well - NLT, but it attributes for heavy customization of engine to enable fluid game flow and art of manageable map sizing, which NLT mastered as no one else, exploiting players interest in solving logical puzzles rather grind on crawling over dull and anemic maps )
Flash (very limited, does not suit for complex games, mostly used by creators with skills in 2d vectored graphics, not many good examples aside publicly available MnF type of games, which nowadays realistic 3D renders make look cartoonish, biting off auditorium of flash games.
Based on that, I wonder if Flirty F and similar, I guess Timestamps can be such sibling, that run on unity actually use its 3D rendering capabilities?
Or use of Unity engine here is pretty much container to link game logic with replay of pre-rendered image series to simulate animation?
Those animations in high res look very detailed, but I have not seen same level of details in “The Twist” so I got assumption that something like DAZ or Blender is used to create high res images, so use of Unity 3D is pretty nominal to pack game into executable?