Some apps look good on both iPad and iPhone. This is called “Responsive Design”. And back when there were only 2 or 3 phone sizes, designers could just design 2 or 3 screens for each screen.
Now, with Dynamic Type, and so many different screen sizes, Responsive Design becomes more important, just like on web.
It really depends. It depends on what your app needs, and what your designer wants.
One approach is never use custom components. This will let iOS layout everything for you. It might not look like how you had in mind, but it will be readable. This is how I like to start new apps, but it seems I never stay with this approach.
A second approach is to use breakpoints. Choose a predefined layout based on the screen height or based on the accessibility text size. This approach is often used on the web. See TailwindCSS for example, their approach encourages you to design for the tiniest screen size you support first, and then add in more elements as the screen size increases.
A third approach is to sit with a designer and talk through all the different layouts across all the different possibilities: translated text, accessibility text size, screen size from iPhone SE to iPhone Pro max to iPad.
In Haptics Studio, I use my preferred layout up to a certain point. But at a certain accessibility text size, I revert to option one: using Apple defaults. Find out more about Haptics Studio
In Underway, we don’t use dynamic text at all, it’s on our roadmap. And on the tiniest iPhone SE, the map screen’s popup was difficult to use. We’ve ensured that the design works well enough for up to 3 digits of minutes for a train arrival. And in the most confined space, the popover on the map, we’ll never go over three digits, since we convert it to the string >1 hr. Find out more about Underway
I’ve also been on teams where they have a pre-built design system, and product designers and developers simply use pre-built components to specify screens. These pre-built components were created to self-adjust depending on how long the text is, and accessibility text sizes.
There’s always eliding the text with ellipses, and adding a button to read the rest. Knowing when and where to do this is an art, not a science.