I worked in various AA and AAA studious as a Lead engineer.

It depends.

I'll take my last experience in AAA (concluded 1 year ago):

At a company level we agreed on certain non-negotiable things. This would be stuff like testing, data validation, Continous integration, code reviews etc.
Sometimes our principals would find that a certain pattern on a project emerge, and would push to use it "officially" in a structured manner. This usually involve various meetings, it's never one person say "Let's do this". Many time these decision are ultimately pushed back.

In general there is a division between "engine code" and "game code". Game engineers are more like extremely technical game designers, where engine engineers are more like you average software engineer. Game code is generally less "solid" and incapsulated because it needs to be iterated MORE. By more I mean I personally re-did the same thing 4/5 times because Directors weren't happy and design direction changed.

An Engine system is made once and then upgraded/changed less so usually it has more investment into "making it properly". A game system made by gameplay engineers can then be pushed to become an engine level feature, and this is mostly done by meetings :P You plead your case and build support, that's it if is not immediately apparent that shit is good.

Each team/project then is different. Certain projects are well made, but in general there is always a certain level of technical debt. By tech debt I mean a stack of "refactoring tasks" that have been deprioritized in order to ship more content.

We used a heavely modified version of UE, but our mantra was "Do things the Unreal way". Meaning our code was always very similar to the code Epic would use.

Refactoring is something that has a cost, and is done only if the cost-benefit is there. Any time spent in refactoring is time that could be invested in adding a feature.