This is a complex question involving several programs.
I can transfer a generic file (meaning one with arbitrary suffix and not necessarily openable by default with any program) from my PC to my iPhone using iMazing on the PC and File Manager on the iPhone. In File Manager I can create a folder, call it what I like (let's say "RRR"), and put the generic file in it (call it "r.zzz"). Fine.
What I need to do is to be able to send such a file to a contact using WhatsApp. The problem is that when in a WhatsApp chat I click on "+" I get an option to send a "Document", and then I get access to various folders but NOT to the folder "RRR". None of the other options, such as "Photo and Video Library", are any help either.
So my question is: how can I send the generic file r.zzz to a contact using WhatsApp?