Yes it is downloading 15GB and also some "useless stuff for other devices".
But this useless stuff is very small, maybe 50MB.
Your next step will be to find the device and kernel sources for your device and place them in the appropriate folders.
You also have to either download the vendor blobs or extract them from your phone.
It also helps to have the cyanogenmod sources for your device as reference. (another 30GB)
After these steps you can try to build Ubuntu touch for your device. There will be a lot of errors and you will have to solve them one by one