OctoPrint is a fantastic Open Source project (just like the marlin firmware), that enables remote monitoring and control of your 3D printer over your network from your web browser. This is commonly installed on a Raspberry Pi single-board computer, hence the nickname “OctoPi”.
DaHai did another excellent video showing how to physically mount, and provide power and USB connections inside the A5 chassis, as well as setup octopi with a camera as well. Click here to see that video.
Note: it has not yet been confirmed if this mod will fit inside the smaller A3S chassis - presumably it will, as the A3S has an external power supply.