Online
An iOS or iPadOS device is online if:
it is turned on (with or without a passcode lock)
it is connected to a data network, such as 3G, 4G, LTE or Wi-Fi
A Mac is online if:
it is turned on
it is connected to the internet using Wi-Fi or Ethernet
An Apple Watch is online if:
it is turned on
it is connected to a Wi-Fi network
it is connected to iPhone
Note: Apple Watch Series 3 or later (GPS + Cellular) is online if it is turned on and has mobile network service.
AirPods are online if:
they are out of their case
they are within about 10 metres of one of your iOS or iPadOS devices
A Beats product is online if:
it is turned on
it is within range of one of your iOS or iPadOS devices (the range depends on the model of Beats product)
If a device is online, a green dot 
appears beside the device in the My Devices list.