Split the output string value

You can utilize the patternMatch function to split the device name string into different columns as follows:

parts(deviceName) =
  patternMatch(replace(deviceName, "_", " "), `{part1:string} {part2:string} {part3:string}`);

foreach device in network.devices

select {
    device: device.name,
    device1: parts(device.name)?.part1,
    device2: parts(device.name)?.part2,
    device3: parts(device.name)?.part3,
}

Underscores in the string are replaced by spaces, since spaces are used by the patternMatch function to parse the string.

Note the (?) after the parts(device.name) function is called. This allows for the condition where the output of the function is null.