For bank templates with the EBP bundle, I see some preexisting templates has fields like "companyInformation" or "taxregistrationnumber". I don't see any docs mentioning that these exist, they only mention stuff like ebank, cbank, payments. Is there some documentation I'm missing, how can I find out what I can actually use in these file templates?

those fields are template specific, meaning that only applicable for those region specific templates. For more info you have look into the documentations for those templates (not in NS documentation) - such as ACH/NACHA, ISO 20022, BACS, CPA-005, ABA etc.