There are several websites for tracking aircraft. Some of them censor some aircraft (like special rich people) so you can't see them. ADS-B Exchange does not participate in this censorship. The primary source of aircraft tracking is from the ADS-B radio on the aircraft. Aircraft may not always have their ADS-B radio on. Such as government drones when they're not near an airport. Learn more about it by searching for ads-b.
Aircraft Tracking on ADS-B Exchange