> Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos

You're making huge and incorrect assumptions here, no? This also happens when your phone is entirely idle... and it randomly changes if you sit still for some time...

Your phone is never idle. Open your adb logs sometime and you'll see that it's doing a ton of stuff all the time. Much of which involves a network connection.

So your phone is basically always doing something, and frequently sending and receiving data when you assume it's doing nothing. By design, radios hop around between channels as conditions change. Another device somewhere outside your house kicked off a big transfer and your device hopper channels to avoid interference. Random atmospheric conditions introduced new noise, or another channel cleared up. This is standard, normal behavior for WiFi, Bluetooth, cellular, and essentially every other type of modern digital radio.

What you're seeing is normal and expected behavior. Modern radios and operating systems are vastly more complex than you're assuming.

How do you know that your phone is entirely idle?