I recommend finding the specific u-blox receiver firmware Interface Description document, but here is the one for u-blox M10 SPG 5.00. Section 1.5 GNSS, satellite and signal identifiers describes how UBX protocol messages number satellites.
By default, the gnssconstellation function models an ideal GPS satellite constellation. You can use an almanac or RINEX file contents to get satellite ID outputs from the function.
