A single large zip file that has been split into multiple smaller files has the extensions .001, .002 and so on. To unzip this file, place all the split files together in the same directory, right-click on the first file (typically the file with extension .001) and unzip it using 7-Zip. This operation is a bit non-intuitive, but 7-Zip is actually putting these files together into a single zip file and unzipping that.
Tried with: 7-Zip 4.65