To split the large sized zip file, use following command:

zip ExistingZipName.zip --out ./TargetFolderName/AnyDesiredName.zip -s 20m

It will create AnyDesiredName.zip along with some more small zips with extension .z01, .z02, .z03, .z04...etc.

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To combine/merge already split files into one Main.zip, use following command:

zip -F YourDotZipFileName.zip --out ./TargetFolderName/AnyDesiredName.zip

Note that, you don't need to specify name(s) of other small zip files (e.g. .z01, .z02, etc)