Who decides what kind of work a citizen may or may not do in capitalism?

The individual decides the work they will do in a capitalistic society. (Assuming there are no physical, mental, health or other issues that would limit work choices.) Education, training, experience and ability are some items that factor into the decision and those qualities may be measured by other people for the work in question. For example if you wanted to be a passenger airline pilot. You are expected to take all the classes and hours of training to prepare yourself for the job. The airline you want to work for will decide if you have the training necessary and then hire you or not. If they don't hire you then it is up to you to get more training or experience and come back later. You can also search for other pilot work that is not so demanding but may fit your skills better.

If that doesn't work then perhaps go buy your own airplane and start your own business where you determine the work you will do.

A lot of it has to do with personal motivation rather than another individual making a decision for you. Many people have 3, 4 or more kinds of work in their lifetime.

In my case the occupations have included machinist, truck driver, printing press operator, print shop owner, commercial cleaning service, landlord and music instructor. All the different work experiences were the result of my decisions. Nobody else.

Hope this helps answer your question.