Hi @mark1234, I am sorry, your migration from classic sp 2013 sites means, that the old XML infopath forms are not made for the new modern JSON script struktures within your modern sites. So in the classic SP Sites it is still possible to refer to old infoPath Forms, but not recommended, because the end is near ;) I recommend, you should learn PowerApps and the Logix behind, because it is way easier to process large amount of data within not only SharePoint structures but so many other cloud storages like Microsoft OneDrive, Dynamics and beyond.
Learn PowerApps from the Pros: Like Laura Rogers or April Dunham. Both have very good youtube channels to learn from them! Greets, Eva.