If someone calls and already knows a great deal about you, people tend to assume that your phone number or other details were sold. That feels like fraud.
A friend you have known for only a few days knows your name, then turns up other information about you through the internet. That might feel like hacking.
Hacking, in the ordinary sense, rests on a fairly plain foundation: access to certain resources and databases, fluency with different kinds of code and software, and some knowledge of vulnerabilities. That is the premise. The actual work is often less dramatic. A vulnerability on a small platform can open a path into its backend. Once inside, permissions and passwords can be altered, and the database holding information about you—or the traces left by ordinary, careless use—becomes accessible. From one small trace, the trail can lead to a larger platform, where more traces are found and the data profile grows. Or those traces, small or large, can be compared against an existing social-engineering database to extract more information.
Hackers still rely on particular internet tools and software, plus careful logical thinking. It is not magic. Someone who never enters the information age, never comes into contact with society, and lives deep in the mountains cannot be found by a hacker in any meaningful way. The internet made hackers possible, and it is also what makes them effective.
A decade or so ago, when the internet was still developing, finding information about someone online was enough to be called hacking. Today the label more often requires breaking into a system and contending with its security defenses. What is described above is probably the common, surface-level understanding.
There are also stories that the real work requires something like Tor, an entry into the onion network. Those are stories rather than firsthand accounts, the kind of thing that makes for dinner-table boasts. It is safer to keep some distance from that world.