How do you implement circular movement in language c?

I'm trying to make a game like Gallagher in C language, and the rest of the elements are all implemented, but they're blocked at the stage of implementing the movement of the enemy. It'd be nice if it was a simple random move, but it's too hard to guessAs I see it, I want to implement the movement of the enemy plane in a circle. When I searched the Internet, they told me to use a trigonometric function or an equation of a circle, but I didn't know how to implement it even though I saw it with a score of 30. I wonder how to implement it in detail.

c function

---