I can't believe I haven't found this until now, but I believe this is the official documentation for the actual form building platform that Survey123 uses (ODK).
There is a good amount of applicable documentation here, including information on repeats recipes tips on question types and XLSForms overall.
Does anyone have any additional info on this? Or opinions on its usefulness?
If anyone finds any gems in their documentation, please post them in replies here!
Well they are both based on the XLSForm spec yes. That is why there are limitations at times since 123 wants to stick to the standard. But for me things like having a space as the select multiple separator was not a great choice by XLSForm and I wish 123 would deviate. But I get it.
As far as the documentation you can find things in there but it does mean that 123 supports it yet so gotta be careful on that.
I have this doc bookmarked https://xlsform.org/en/ref-table/