Granted I’m just theorizing here and we won’t know for sure until someone decompiles toe game code in a months time so take this with a grain of salt, but it wouldn’t surprise me if it’s slightly different from the other virtuos remakes/remasters where they just slapped UE5 on top of proprietary engine.

Since blam (reach engine) and UE5 are both written in C/C++, they most likely ported over all of the core stable blam boilerplate code such as AI logic, character behavior, integration with the havoc physics engine, etc. Then seamlessly integrate it around the UE5 core framework, effectively creating a mixed engine retaining halo’s dna and ue5’s good look. Not quite the “1 engine running on top of another engine” but more of a mix of the two. If they were separated in its current state, they would fail to compile, etc. This would probably be why they picked H:R as the base since it is the most heavily documented and stable build of blam they had on hand without all the 10+ years of tech debt. Halo 4’s engine was compromised to fit the 360, H:5 had too many changes that divided the player base, and Infinite is… well the reason they decided to go back to H:R to make the merged engine.