> Then, there’s a random byte like 0x25 or 0x07 followed by the column data – the rest of the columns are string types so they’re all stored in UTF-8. If you know what these inter-column bytes mean, leave a comment below! I can’t figure it out.
Next paragraph mentions TOAST and this byte is related to that. The low order bits (on little endian platforms) determine whether the value is stored inline (00, first 4 bytes are total length), is stored in TOAST table (11) or is shorter than 127 bytes (01 for even length, 10 for odd length, the total length is first byte >> 1). So for 0x25 you get 01, so length is 0x25 >> 1 = 18, which is that byte followed by "Equatorial Guinea".
Edit: the reason why endianness matters is that the same representation is also used in memory and the whole first word is interpreted as one length value. The toast tag bits have to be in first byte, which is most easily done as two highest order bits of that word on big endian. That means that it is placed in the two highest bits of the byte.
Next paragraph mentions TOAST and this byte is related to that. The low order bits (on little endian platforms) determine whether the value is stored inline (00, first 4 bytes are total length), is stored in TOAST table (11) or is shorter than 127 bytes (01 for even length, 10 for odd length, the total length is first byte >> 1). So for 0x25 you get 01, so length is 0x25 >> 1 = 18, which is that byte followed by "Equatorial Guinea".
Edit: the reason why endianness matters is that the same representation is also used in memory and the whole first word is interpreted as one length value. The toast tag bits have to be in first byte, which is most easily done as two highest order bits of that word on big endian. That means that it is placed in the two highest bits of the byte.