A game engine is really a project that scales really well with the size of the team. There are a monumental number of things you could put into one so they will

A game engine is really a project that scales really well with the size of the team. There are a monumental number of things you could put into one so they will swallow any number of engineers you throw at them. For comparisons you have PICO-8 which is an intentionally super limited but fully integrated game engine made by one person. Then you have Unity which is a very featureful engine, ported to every commercial platform with a vast toolset and side offerings which arguably has lost sight of a bunch of the game engine part of things because they have the capability to spin so many plates and a business incentive to drive revenue by getting people to sign up to extra services.