Aks HN: How do you excel in ambiguous environments?

The higher the pay, the more it is required from the engineer to "just figure out problems and their solutions".

For example: A StartUp is hiring the first 5 engineers. Some of the first ones come in, and see: "Oh, the structure of the code is not quite as good" or "we should really implement x instead of y here for these reasons."

What if you struggle in these environments and want to learn how to excel there? Is it just applying past experiences to a new situation, or can you learn to look for solutions?

TL;DR: How to find problems and solutions to them in the codebase (and product) without a vast prior experience in that specific environment or industry.