The strange thing about the bug is that it wasn’t directly powered. It was actually a resonant cavity that only worked when it was irradiated with an external RF energy. Most of the video is background about the bug, with quite a few details revealed.
It goes to show that technology does not have to be complex to be effective. In this case, the simpler, the better.
The video at the link below has a lot of background of how it came about, as well the recreation exercise for the BBC. But the article also contains some links out to further sources on the reasons why the bug actually works.
And if you are not used to a colloquial English accent, the word sounding like “book” is actually “bug” 
See https://hackaday.com/2023/03/25/recreating-one-of-historys-best-known-spy-gadgets/
source https://gadgeteer.co.za/recreating-theremins-bug-how-the-soviet-union-spied-on-the-us-embassy-for-7-years/
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