It's a mesh network that relies on beaconing to devices with Bluetooth enabled or more preferably Apple devices that are UWB-capable (U1 chip) which in return gives more precise tag localization. You can track tags from your either phone or tablet; these so far aren't visible on iCloud page IIRC.

I've got one but it seems to be activating only when Apple devices are around

Similar devices exist around - Samsung for example has own line of such tags

https://en.wikipedia.org/wiki/Bluetooth_Low_Energy_beacon

https://en.wikipedia.org/wiki/AirTag