Writing does not have to be a career thing, but one can spend one hour on it a day or two or three hours.
A session of writing sounds to me like I have accomplished something within a period of time.
It’s a session of writing, even one session a week, but for every week, not a one-off. This keeps one going. It may be a session every two weeks.
I think one can be a writer with a little background in some subject or occupation and use that experience and knowledge to approach publishers for assignments and work or submit material.
One can build up a file of pieces of work in one subject. At the moment, mine is simply reading and thinking about the Bible, which has led to devotional writing.