Apple provides an article entitled, Import your iBooks Author book into Pages for Mac, the describes how Pages v10.2 handles existing iBooks Author documents when you open them.
Provided that you had iBooks Author prior to the Aug 1, 2020 download cutoff, and that you still need to generate documents with iBooks Author widget features in them, I recommend that you continue using iBooks Author. Pages v10.2 does not feature any iBooks Author widgets, or their replacement functionality. These may be arriving in future releases of Pages, but the adverbs surrounding that timing are known only by the Pages product team.