An interesting experiment would be the same thing you are doing but isolated in a note taking app to discard the google keyboard you’re using. Also, it would be interesting if you can use e.g.: proxyman (available directly on iOS), or some proxy on your PC to intercept your network traffic and then try to reproduce while blocking/allowing some domains. Especially, blocking all google domains, then facebook domains, etc. If you have a Pi-hole set-up doing that at dns level may be easier.

I’ve never been able to reproduce these experiments. But keep in mind that I’m european and my WhatsApp app is slightly different - it is a version from WhatsApp Ireland (instead of WhatsApp Inc) which shares less data with third parties, the privacy policy is also slightly different for the european union.

Edit. Another idea: try to reproduce while disabling predictive text on your keyboard.

Who is they? Google or meta?

Google are definitely collecting data from gboard.

That may not be directly shared with meta but is likely to get indirectly shared through overlapping advertising identifiers. They won't be openly sharing your text, but they will be scanning to and flagging you as having interests in something in your text or something related to what you said, then sharing that with advertisers.