These are visible not just by anyone who can receive, but pretty much everything on APRS ends up online. Most areas have an igate that forwards messages from the radio to the Internet. https://www.aprsdirect.com/center/40.55354,-104.97842/zoom/1... is my favorite site for seeing what is going on in an area and where digipeaters and igates are. https://aprs.fi also works.
Some areas have one that works in reverse, taking what is seen on the internet and relevant to the active stations in the area and broadcasting it out over radio.
When I started using SMSGTE I had great success getting a SMS from my radio to a phone, but only 1 in 10 replies from SMS made it back to my radio. I put up a gateway for my area and so we have some decent bi-directional coverage.