@KarvaRausku Those games are less complex from a code perspective at least: simpler movement, less features (items, abilities, interactions, etc), hitscan weapons, small maps, less players per map, etc..
As for the Apex legends being a Titanfall mod turned into a game, I can say the same about Fortnite BR and there are tons of other games that get developed this way, there's almost none developed from absolute scratch nowadays.
The assumption that the devs don't know what half the code does can be applied to every programmer in the world nowadays. For example, I'm currently working in Unity, but I have no idea what happens inside the engine, and that is not my job anyway, there are dedicated devs working on Unity development. It's the difference between using tools and developing tools.
Anyway, Respawn devs have a modified engine, which means they know more about its features than they would know about Unreal engine, for example. Also, they have the experience of not only Titanfall, but Call of Duty, as some of the veteran devs come from Infinity Ward. They're some of the most experienced FPS devs around.
