gps can track you to the exact location, even texts or posts sent need to be processed through somewhere, before going to the intended person. Any app that asks for access and to what, on your phone and agree to the terms before downloading. The people running apps can see what you're doing on it, if it's agreed upon when downloading and see what it has access to. Honestly I was a little disturbed when I found my texts on my phone became linked with my facebook account as well. Others can't see but facebook can. Not sure when facebook did this but I don't like it. People with Samsung, might have a device with something called Samsung Cloud, that automatically stores every picture, video or data taken and uploaded to it as a backup storage but that would be the same for anything similar, such as iCloud. While a nice thought. Someone's seeing all of it, is my thought.
I don't really think about it though, normally. In the modern age, it just can't be avoided.