v4: 122 random bits. The classic UUID; supported everywhere; no order.
v7: first 48 bits are a Unix-millisecond timestamp, the rest random. Sortable by creation time.
v7 is much friendlier to B-tree indexes (Postgres, MySQL): inserts hit the rightmost page instead of scattering writes across the index. Use v7 unless you specifically need unguessable IDs.
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