My uncle will travel and stay at serviced apartments (whose structure and interior he must not change), across Canada for work where the tap water is normally safe, but he needs Reverse Osmosis (hereafter RO) water for health reasons.
APEC's portable countertop RO system's Diverter Valve (DV) must be connected to the faucet's Male Threads (MT), and so does not help, because even if the apartments' faucets use MT, the MT's size may not match the DV's.
That's what that thing is! Sorry, yesterday I found a mysterious little tube that had 2 different threads of slightly different diameters on it. It's the gadget that solves your problem. It's for fitting a device which has a female thread (wants a male on the faucet) to a faucet which has a female thread.
They are listed with names like "faucet adaptor" or "dual thread adaptor".
They typically come free with devices which go on faucets, and are either used in the first installation and left behind... or promptly lost or thrown away with the packaging. Any decent hardware store should sell them.
"Adaptor kits" also exist which claim to cover every possibility and include washers:
Now, when you leave, you will probably find the adapter has attached itself to the faucet and will refuse to come out. In that case, the original aerator won't fit anymore. Just get a simple aerator that fits it, install that, test and walk away.
In a general sense, however... The answer is get good at adapters. They make many more shapes and sizes.