Open Setapp and click "Available on iOS" in the sidebar. You'll see all apps that have iOS companions; click any app to open its page.
You can also find iOS apps while navigating through the entire Setapp's app collection (here are some hints on discovering apps). A small icon in the lower-left corner of an app icon shows that this application has an iOS companion.
Do all apps in Setapp have iOS versions?
No. Even when an iOS app is on the App Store, there might be cases when it's not available in Setapp.
If an app's icon doesn't have a small icon in the lower-left corner, or there is no "iOS app" button on the app's page, the app has no mobile companion in Setapp.