One way to build your writing skills--a way that I would argue is necessary if you ever want to write original fiction for publication--is to write from the point of view of, and with the focus on, a wide range of different characters.
it's really easy to fall into a rut when writing the same character or characters all the time, or even the same type of character all the time, where characterization tends to become muscle memory as much as anything else. You know what that character will do, so you know what characters of that type will do, so you know what characters will do, so that's what your characters do.
And when you don't have to think about it, you don't build--and can start to atrophy--those muscles required to do detailed, specific, engaging character building. What does it mean for this character, in this time, to do or experience this thing. What are the myriad of things that have built your character up to being who they are, and how do those things (individually and in aggregate) impact the choices that they make, the actions that they take, the reactions that they have, and the people that they engage with.
What can end up happening--and I see this all the time in published fiction--is that authors end up only being able to write 2-3 character types of each gender, and it all feels a bit samey.
Without opening a book by so many authors I have read, I can predict with a fair amount of accuracy what most of their characters will act like, because it's kind of the same across the board. Even when they start distinct, they end up drifting towards the same personality/character types like carcinization.
Writing from the point of view of/focusing on a range of characters (especially if they are different genders, of different backgrounds, with different wants and fears and habits and interests and personalities) forces you to actually be specific in your writing, if you want it to be any good.
Your 15-year-old B-student who really wants to spend their time playing rugby shouldn't sound like your 45-year-old businessman with a penchant for collecting Star Trek action figures who is trying to plan the perfect anniversary for his wife and neither of them should sound like the 23-year-old who spends their time going out at nightclubs and showing up a little bit hungover at work and worrying about finding a job that will let them move out of the apartment they're sharing with three other people.
Practice, and then practice some part, and then keep practicing. Write different characters, ask yourself if you're writing a character a certain way because you think they would be that way or because it's just habit, and be specific.
