I actually flew into Kyiv last year and had the exact same questions. The two airports are at totally different distances from the center, so your transport strategy really depends on where you land. I almost overpaid for a random taxi until I found this resource. It breaks down the best ways to get into town from both Boryspil and Zhulyany, including the express train schedule and which apps to use for the best rates. It saved me a lot of stress (and cash) right after my flight. I’d highly recommend checking it out before you land so you have a solid plan ready!