Whether real or imagined, I think the fear is that you won't know what info is sent.
And that's the point.
Supposedly, information is tracked, sent to a database and stored there for whatever metadata can be gleened for some nefarious use at a later time.
We know what Google/Facebook/Apple (yes, even Apple - don't think they don't keep gobbs of data on users as well, they just don't sell it the same way) does with the data they get from us, so the trick is to not give them much in the first place. For example, with Google, have a Google account if you wish, but don't use Gmail (you don't need a Gmail account to have a Google account - any email will do - even an outlook.com email). For Facebook, use only basic information anyone would know just looking at you, and maybe the schools you went to if you want. But don't fill in those favorites lists (tv shows, movies, music...), political and religious affiliations, and all that stuff; because that's where the social engineering attacks and ads come from.
My feeling is that this fear of Chinese companies has some foundation in the fact that these companies do have ties to the government. They all have to to stay in business within their boundaries.
Whether or not you want to believe that Beijing actually has these tech companies install extra chips in these devices that intercept all your data and send it to a server in China for storage and later analysis, is up to you.
Except for some anecdotal talk several years ago of such "mystery" chips on some server equipment from Huawai, and a few security experts finding some Chinese apps on some Chinese phones sending user data in the background to known Chinese IP addresses (a TCL weather app in one case), not much else has ever been really verified.