Human composition of chemicals has evolved over a long period of evolution. Choices stem from this composition. The composition however is not perfect. It has evolved by random mutations with best mutations surviving the environment. Composition is made to learn which decisions and choices come to a good outcome however it is not perfect (falling pray to drugs) or it does not always adaot to fast evolving environment (being addicted to social media).

But the decision and choice itself comes from chemicals of course. It is like asking which was first, the bits in the computer or the calculation it produced.

There is no scientific answer to that question. Neither of those things exist in separation in reality. They are only separated in our mind.

Some questions can be answered with science, others cannot. Both types of questions have meaning.