If you enable Automatic Downloads, your purchases from the iTunes Store should be automatically transferred to all of your devices. You may find the following article helpful:
Alternately, you can download just the purchases you want manually:
So many people think it is complicated to move music from phone to iPad. We always copy the music to computer first and move them to iPad. Actually, we can transfer music from phone to iPad directly now. And just one click can we transfer the music rapidly.