1. Open the Terminal.app from Applications ยป Utilities
2. Use the following command, adjusted to your needs and ZIP-file location(s):zip -s 0 ~/Downloads/splitted-zip.zip --out ~/Desktop/singlefile-filename.zip
3. Once you have created the single ZIP-file, use the following command to unzip it (again, adjust to your needs):unzip ~/Desktop/singlefile-filename.zip
(from https://swissmacuser.ch/unzip-combine-splitted-zip-file-archive-macos/ )
However, if you are dealing with a set of zip files that do not include numbered file extensions (foo.z01, foo.z02, etc) and are simply multiple zip files that should be unarchived together into the same directory (such as a split Google Drive zip archive download: foo-001.zip foo-002.zip etc.), the following should work:
unzip '*.zip' -d /path/to/unzip/destination
(from https://superuser.com/questions/365643/how-to-unzip-split-files-on-os-x/517758#517758 )