Lets assume we have a device, which contains device1 and device2,
and each of them contains part1 and part2, so lets say we have
connections like

device1.outpin1 -> device.inpin1

devices1 is:

device1.interminal1->part1.inpin1

part1.outpin1->part2.inpin1

part2.outpins->device1.outterminal1


devices2 is:

device2.interminal1->part2.inpin1

part2.outpin1->part2.inpin1

part2.outpins->device2.outterminal1

Two instances of the same part is used and the device will look like:

INTERMINAL -> part1_1
part1_1 -> part2_1
part2_1 -> part2_2
part2_2 -> part1_2
part1_2 -> OUTTERMINAL

Any application can be made on one-layer device. The goal of the device is to hide complexity and make reusable part and it is not designed to perform any function (at least at current stage of buildap design). Quite similarly like any software can be coded in one big method (as in early days of software programming).