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A version 4 UUID is 122 random bits written in a shape everything agrees on. These come from the browser’s cryptographic random source, so they are as unguessable as the ones your database would have made.
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The thirteenth character is always 4 and the seventeenth is one of 8, 9, a or b. Those are not random: they are the version and variant marks that tell anything reading the UUID that the rest of it was drawn at random.
That leaves 122 bits of actual randomness — enough that you can generate them all day without ever expecting to see the same one twice.
Nothing is requested from a server. The UUIDs are built in this tab from the browser’s own random source, which is the same source a password manager uses.
Which also means nobody, including this site, has a record of what you generated.
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