Whatever code you are handy with. You'll need land generator tools and profiling tools - and picking those out and developing skills with them is a step toward understanding scale too.

In the course of building those prototypes, you are learning something about scale too... what if I spawn a bunch of threads to handle this task at this point? What if instead I put each of those tasks on a queue and have a task handler? What if I take this engine piece and put in on separate hardware? What if I have a bunch of these engines running in parallel? What if I make this change to the DB connection? What if I remove the dependency on the DB entirely?

It's easy to say a lot of this stuff is basic computer science you should have learned in school, but that's not reality. There's a lot of emphasis put on algorithmic complexity and efficiency, there's a lot less put on systems design and architecture. You kinda have to do it in the wild to learn it.