Your own network
Bluetooth links the devices around one person: a phone, earbuds, a watch, a keyboard. That is a personal area network (PAN). The links are short radio links, about 10 m for most devices.
All Bluetooth radios use the 2.4 GHz band, 2400 to 2483.5 MHz, which Wi-Fi and microwave ovens use too. Bluetooth Low Energy (BLE) cuts it into 40 channels, each 2 MHz wide.
Find and connect
A new watch sends a short advertising packet: "I am here, you can connect". It repeats it on the three advertising channels, 37, 38 and 39. The phone is scanning, so it hears one copy.
The phone answers at once on the same channel with a connection request. It sets when the two will meet and which data channels to use. Now the phone is the central and the watch a peripheral.
The first time, they pair. Each sends a public key and both work out the same secret, which never travels. You check that both screens show the same 6-digit number. The saved key encrypts the link.
Staying connected
Connected, the two meet at fixed times, here every 30 ms. At each meeting they switch to another data channel. If one channel is noisy, the next meeting happens somewhere else.
The keyboard connects the same way. To pair it, you type the 6-digit code the phone shows. After that, every key press is a tiny encrypted packet, sent at the next meeting a few milliseconds later.
Classic audio
Music needs a steady stream, not short bursts. So earbuds often use Classic Bluetooth (BR/EDR), the older, faster kind. The phone keeps sending small audio packets, and the earbuds play them in order.
Classic uses 79 channels of 1 MHz and jumps to a new one 1600 times a second, in an order both sides know. Adaptive hopping skips the channels a nearby Wi-Fi network is busy on.
The phone runs all these links at once and sets the timing for each. A Classic piconet can have 7 active peripherals. Walk more than about 10 m away and the links drop until you come back.
What is going on, line by line
;; step 1/10 · pan · local ;; at the earbuds, watch, keyboard, phone (central) personal area network (PAN): devices around one person phone 00:00:5e:00:53:01 central earbuds 00:00:5e:00:53:20 peripheral, Classic (BR/EDR) watch 00:00:5e:00:53:21 peripheral, Low Energy (LE) keyboard 00:00:5e:00:53:22 peripheral, Low Energy (LE) address (BD_ADDR): 48 bits, like an Ethernet MAC power class 2: 2.5 mW (4 dBm) → about 10 m class 1: 100 mW (20 dBm) → up to about 100 m radio moves at about c: 10 m in about 33 ns
The figure is slowed down: a real LE link may meet every few milliseconds and a Classic link hops every 625 µs. The watch name, the 6-digit code 314159, the hop increment and the Classic hop order are illustrative; the real Classic order comes from the central's address and clock.