No orientation readings. The overlay can't follow the phone. In Chrome, tap the icon left of the address → Permissions → allow Motion sensors, then reload.
Point your phone at the sky to see which planes are up there: airline, flight, type and distance, from live ADS-B data.
Uses the camera, compass and location. Your position, rounded to about 1 km, is sent to adsb.lol to ask for nearby planes. Flight data: adsb.lol (ODbL); routes: adsbdb.com.
Live positions come from adsb.lol, a free community ADS-B network (data under the ODbL licence). The feed doesn't allow calls from a web page, so live traffic needs the Android app; the web version shows simulated traffic.
If the boxes sit beside the planes, line up a plane you can see: tap its box, then Align compass on it. On a sunny day, Calibrate with sun works too.
Across the long side of the camera image. If you calibrated it in Sun Path, use the same value.