Version 4 UUIDs from your browser's secure random source, in whatever quantity you need.
A version 4 UUID is 122 random bits plus six fixed bits that mark the version and variant. This page takes the random bits from crypto.randomUUID where the browser provides it, and otherwise fills sixteen bytes from crypto.getRandomValues and sets those six bits itself. Either way the randomness comes from the operating system, not from Math.random.
Not guaranteed, but the odds are absurd: you would need to generate billions per second for decades before a collision became likely. For practical purposes they are unique.
None, in practice. GUID is Microsoft's name for the same 128-bit identifier format, sometimes written in braces and uppercase — both options are available above.
It works and avoids coordination between systems, but random UUIDs scatter writes across an index. Where insert performance matters, a time-ordered identifier such as UUIDv7 behaves better.
Yes. Once this page has loaded, generation needs no network at all — everything happens locally.