My vote is that it screws onto the faucet threads. But I’m also guessing that the previous owner just added that ad hoc rather than it being an SMB option due to the lack of an on-off feature on the shower head, as someone else noted. I added a shower to my old van using a kitchen sink sprayer; I had removed the sink and repurposed the water line. You could also easily add a t-fitting and a valve to achieve the same thing with a sink.
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