The main method of sending data is advertising. A device repeatedly sends out a small amount of data every defined advertising period. This data can be, for example, a unique id, a URL or sensor data. Any Bluetooth LE device can scan for and see the advertising. At this point, there's no connection. Anyone can see the data and the sender doesn't know anything about the receivers.
Once a receiver sees the advertising it can optionally connect via GATT to see what services the sender provides and then connect to the services to update data, for example to configure the sender, or get data that the sender perhaps didn't show in the advertising due to size or privacy. There are standard services defined by Bluetooth profiles. It's also possible to set up services that only allow change of data after the correct password has been sent to another defined service.
Consumer and industrial devices, for example fitness trackers, heart rate monitors and industrial fridges, send out identification and/or status in the advertising. Android and iOS apps connect to configure the devices or extract further data.
Beacons primarily send out identification information and beacon manufacturers' configuration apps connect to change beacon settings such as identification, transmit power and advertising period. The identification can be used by consumer apps to trigger content based on location and by gateways to provide for server based locating systems such as real time locating (RTLS).
Sensor beacons, used for IoT, measure quantities movement (accelerometer), temperature, humidity, air pressure, light, open/closed (magnetism hall effect), physical proximity, fall detection, smoke, gas and water leak and present this in advertising or via GATT connection.
Tracker beacons are different in that they predominantly work in connected GATT mode and cause an alarm in an app an/or in the beacon when the connection is lost.
Receiving devices experience varying received signal strength, RSSI, due to transmissions from sending devices that can be used as a rough indication of distance from sender to receiver.
More complex architectures are also possible. Bluetooth mesh uses a device that is advertising, scanning and optionally servicing communication via GATT.