Z file
.z
files are a type of compressed archive in Hey! Pikmin.
.z in Hey! Pikmin is LZ and Huffman encoded with a 4 byte alignment.
Tools
Use the Yoshi's New Island decompress BMS script with QuickBMS.
Alternatively, for recompression, use the pinned script in the Hocotate Hacker discord's #hey-pikmin channel.