Most developers including bungie run off of a modified game engine, Then they ad certain properties to it like physics and particle effects as well as lighting. and what not, The call of duty franchise (even the newest one) runs off of a HEAVILY modified Quake 3 engine (which is very old). Valve still to this day uses their source engine, which was first being developed near the time of the quake 3 engine was launched. Most likely they will run off of of the same stuff they've been working with in the past. But will further ad to it (like they did with reach) Which essentially creates a new build. Don't know every detail but that should explain the basics.
Still crazy though how games have jumped from a couple hundred polygons (which resembled a regular person a robot) to thousands of polygons. in art direction most developers are Now aiming towards a more "hand crafted" feel (that's the terms most people say now). This is good because now for developers for they no longer are as limited (especially as developers tools are becoming more easier and easier to use) with the exception by their imagination and Creativity. Which has, in my opinion been the actual problems with most games now. Creativity . But that isn't new news, mmkay? got all of that?