Testing buildings without connecting power?

[Starles]

I see some Youtubers doing guides where they have buildings running even though it is not actually hooked up to power generators of any sort. They literally have wires hanging off in mid air. How does one do that?

Additionally, previously there was a debug command called Gameplay test mapped to Shift F2 but now that opens the Automation overlay instead, and gameplay test is no longer available in the options control settings.