Earlier quoted context omitted.
Not sure, but isn’t unity in this case covering your need for some tools, like rendering, assets management and so on, but maybe not all - i.e deferring to them the physic. Or so would I say assume ?
I heard from someone who actually works in the industry (as engine developer) that companies usually don't use Unity out-of-the-box and write their own tools and extensions (e.g. memory management) and change parts of the engine as the needs vary greatly depending on the game they are making. Not sure how common that really is though.
I've been using Unity since 2009, and have been a lead developer and CTO in several different companies. I have never heard anyone writing their own memory management for Unity. May be you're talking about object pooling?