how to specify the unzip target directory

There is no such option, at least not for Info-ZIP. You have to create a directory projx yourself, move/copy your files into it and then pass it to zip.

If the directory already exists then just pass option -d to unzip to specify the target directory for extraction.

To do that there is an another way. You can flatted the entire contents of the zip file using following method. Target directory is "projx" and execute following command.

cd /projx && unzip -j /path/to/myzipfile.zip

Option -j is called "junk paths" and just dumps each file into the current directory instead of extracting any directory structure.