Uhhh that's not accurate. It shows local aircraft that are BROADCASTIG ADS-B. To assume all aircraft are is a huge mistake and one that could be painful and expensive.
- Military may not be broadcasting.
- Law Enforcement may not be broadcasting.
- Small manned aircraft may not be broadcasting.
- Not all areas in the National Airspace System require ADS-B.
Also keep in mind that unless you're near an ADS-B receiver some aircraft (if they are low) might not be showing up due to terrain issues.
I'm sure (very hopeful) that it's gotten better but just a few years ago it was estimated that any given moment FlightRadar24 was only showing about
30% of aircraft at any given moment flying in the US NAS.
I've been a long time customer of FlightRadar24 and use it every single day but it's important to know it's only an add-on to situational awareness and very far from an absolute end-all for local Air Traffic notifications.