You can think of a Game Engine as a framework of common things that are needed to build most games.

Before Engines were commonplace, any company that wanted to build a game would have to do things like learning to talk to the computer's graphics systems and writing huge swathes of code to make that work. Basic stuff. And they'd have to do that for every game they built, which was massively tedious and complex.

So any established company would probably have a whole load of engine code they re-used for every project to save time and effort.
Then some bright spark got the idea to market that engine-code to other companies.
Wrap it up in a nice user-interface, provide all sorts of tools, and now you have companies using Unreal or Unity or whatever other system they like. The complex deep-code is done, and rarely needs to be touched.

Nowadays very few games companies bother to roll their own engine-code, it's way easier to buy a licence for Unreal and just build off that well-established and understood framework.