n versus v is determined by the byte order. In the https://libgeos.org/specifications/wkb/ specification mentioned by swl, there is a section on Byte Order which mentions a flag stating the order used, and http://www.geopackage.org/spec120/#gpb_format says this flag is the lowest bit in the "flags" field (second C in the unpack template), so this determines whether to use big-endian (n and N) or big-endian (v and V). Luckily, this flag is a single byte, so that it itself isn't affected by byte-order ;-)

Looking further at the flag byte, I see the the next three bits tell you how many doubles (probably f format, from Perl 5.10 upwards you can distinguish between big-endian "f>" and little-endian "f<") are in the double[] envelope array at the end of the GeoPackageBinaryHeader.

Sorry I'm not really into this, so I can't dive very deep into it. Depending on what information comes up in the discussion, it might trigger me to go looking deeper, but don't bet on it :-)