I have seen people say they do writing in their free time. But I dont understand what they write about. Do you just pick a topic research about it and then summarize it. Is it whatever comes into the mind?

If someone asks me to write about something, I would probably write about Tech but even that I can not do in great length.

What do you write about and how? Do you copy certain writting style from great authors? How do you increase your vocabulary (i think before writting one must read a lot)