What is the device in the garden in the first episode?
The light novel doesn't give much context about the machine, and just describes it as a 小さな機械 (lit. small machine/apparatus).

One assume by the location ns accessories (hose) that it's for watering the garden. Since the inhabitants of this city are all hikikomori-types, it would make sense for machines to do all the work, life watering the plants.