What kind of device is being used around 06:22 into the video to check the light bulbs? Only one terminal is connected and the light turns on. It's like connecting only positive terminal of a battery to a light bulb and turning it on. Could you please help?
It will be some kind of high frequency source; current is passed due to capacitance - think of a plasma ball and such, with a central electrode and arcs outwards from that.
Likewise, putting a transmitter near a fluorescent lamp or the neons some CBers used to put on the ends of their can antennas...
It looks like some gadgets are sold specifically as Neon tube testers, according to Big Clive: