Keys are USearch’s 128-bit identities, written as integers below 2¹²⁸ and crossing as sixteen bytes, most significant first.
That is what IntegerDigits[key, 256, 16] renders and FromDigits[bytes, 256] reads back, so an identity crosses at full precision whether it started as a machine integer, a UUID, or a number too wide for either.
An integer identity is the encoding of a number below 2⁶⁴, so it parks in the low eight bytes with the rest zeroed.
A UUID is the number its hexadecimal digits spell:
uuid = FromDigits[StringDelete["5f9c2b71-8e4d-4a63-b2f1-0c7a9d3e6b58", "-"], 16];
DenseInsert[index, {uuid, 43}, rows];