The format I work the most with in the iPad is PDF.
I import PDF files into Concepts, to scale, annotate and sketch on top of it.
I import PDF files into Affinity designer and create presentations.
Sketchup should export scaled pdf files of our plans, sections and elevations, so we could create our presentations on the go and use the amazing and pro grade vector and raster illustration apps available to ipad to work creatively on top of our model views and linework.
To do it, I have to move my models to Desktop, work on them, send them to Layout and then back to the iPad to paint on top. That’s another reason I have discarded Sketchup for iPad for modelling purposes and use it mainly for presentations and sketching on top of models.