Do people shape culture or does the culture shape the people?
People shape culture, but not exclusively, it depends on the circumstances...
In a startup the founders and initial team they hire do the shaping. In the vast majority of cases the culture develops by default and the team are largely unaware of the shaping that they are doing.
If your personal values are similar to the values of the company, and the company has a strong, functional culture, then you are fertile ground and the company's culture will shape you. Join Amazon and you will be shaped by the culture... or spat out if you aren't
If you are young and you join a company with a strong functional culture (or a weak dysfunctional culture) it will shape you.
Join any large bank, and if you are in it mainly for the money, the culture will shape you. You will learn to fit in, play the game and make as much money as possible .
If you are a CEO brought in to turn a business around you will shape the culture - your new-broom-sweeps-clean first actions will often be to fire a bunch of people and hire people you know and trust to deliver on the culture you want to implement
There are 3 ways to break your company's culture. One of those is to hire people who don't match the values of the company. The negativity/toxicity produced by these people will shape the culture.
One brilliant jerk who isn't fired soon enough will shape the culture.
Your culture is the one sustainable competitive advantage that you have complete control of, so it makes sense for you to shape the culture you want to build