What are the privacy implications of using the InBrowser or InApp share buttons to share sensitive data to a private messenger such as Signal? What information does the application or website that you’re sharing from see?
Another similar question: When you add a Google Calendar to Apple Mail, what information is shared between the parties? Can I assume that both Apple and Google now obtain my data or does remain with Google only?
That heavily depends on the App/Site you’re sharing from.
For example, Instagram adds “igshid=...” to the end of links, so that when somebody else clicks on it they know it was you who shared it, and many other services out there do this too, where they have “share IDs” to figure out who shared what unanimousy. YouTube recently introduced this too, for example.
So whether data about this is collected, and how it’s used, largely depends on the privacy practices and policies of whatever app/site you’re visiting.
To give an example of how this affects a conversation on Signal for example, if you share a YouTube link to a friend over Signal, they won’t really know if you’ve sent it, to whom, over what messenger, but once the other party clicks it (or if you have link previews), they will know it’s been used in some way, usually by the other party in your conversation clicking on it.
I haven’t heard of whether Apple does this, and people are currently freaking out over this exact problem with Outlook (where it creates a copy of your Gmail on Microsoft’s servers when trying to sign in with Gmail), which makes me assume this is not the norm
In other words, looking at how people react to something like this having happened in big fashion recently, I don’t think much is shared other than the fact that you’ve connected to a Google Service using an Apple Product and/or Software.
To add to your answer, a user can remove tracking/UTM parameters manually from the link (the link will still be fully functional, of course, and tracking parameters are pretty easy to recognize in links), or here are some good tools for that:
To remove bad stuff from links before sharing
A similar, more comprehensive tool