Designing mobile apps and handing them over to development in a way that’s responsive to all screen sizes?

I design stuff in Figma, for an iPhone 8, based on pixels. But we need to find a way to implement it in Flutter in a way that the design fits all the other screen sizes. One way we figured out so far is calculating the ratio of every element in relation to screen size… but that can’t possibly be the best practice, and it takes forever. Any insights, ideas or resources that come to mind?