A simple low-cost ADS-B decoder can be set up as shown in the image below.

ADS-B signals can be received and easily decoded on a home PC without expensive equipment. An inexpensive SDR with an appropriate antenna and a computer, even a Raspberry Pi, with free ADS-B software can be used to receive ADS-B broadcasts and decode the data.
Here is an example of text output from a dump1090 program:

There are also a number of commercial ADS-B software packages that can be purchased for a turn-key installation of a personal station.

Those websites that publish ADS-B information depend on lots of private ADS-B monitoring stations scattered all over the world taking part by reporting their data which is then aggregated and subsequently shared.

References:

• PiAware ADSB Software for the Raspberry Pi  🔗 PiAware on GitHub
• FlightAware Flight Tracking  🔗 FlightAware