It will take a while of playing, to get legendary you just need two purples of that color for the S rank item. You can merge the non-S rank purples into the S-rank (I did spend money on the premium daily pass though for more gems) So to get legendary, you would need two hearts of the white dragon, and four other purple hearts non S rank to feed into them. This means though that for the next rank, you would need four Purple S rank hearts, eight regular purple hearts to boost those four, and 24 regular purple hearts to create legendary hearts to boost the legendary S rank hearts you made to +2 to combine with each other. It's a lot and will probably take months

It should be automatic, I don't think I had to do anything

It uses your Google play account to save data