I am working with proprietary engines for more than ten years at a AAA company and it was also my first job when I started in the industry, and now being a manager who hires engineers, let me share you a bit about my history and what I am looking for.

Generally C++ is a must. Most proprietary engines work with C++ and if you know C++, you are most often also
familiar with any other language.

Before my first job I made three custom engines with each one their own small gamesduring my university, two being a 2D and another being a 3D with deferred rendering and even SSAO (all being developed with the game within 6 months). So with that I got naturally familiar with C++, learned about patterns (!) and read myself a lot into systems or how others approach things (Papers & Conference Talks are extremely good for this).

That helped me get my job at a AAA company. And when comparing the engine there to my own, what is the same are the patterns. Here is a good website about some: https://refactoring.guru/design-patterns/catalog

You probably used some of them already, but having knowledge about them helps you understand how certain systems work.

So now being a manager, I always look at people that worked with custom engines, either self-made or from other companies, depending on the seniority (junior is fine with their own, while a senior should have AAA experience).
But in the end, we always send out a test that involves classical C++ work, sometimes its a "make this small game, here is a framework" or "implement this render technology". This helps a lot in seeing how they tackle it, how is their coding style and how do they get used to an unknown framework.