Final Sprint: Metropolis Cardigan

My first knitting project:
And here I would like to ask the knitters among you: What do I use best to sew on the buttons and does it make a big difference if I put a seam on the inside of the sleeves instead of knitting them in rounds? As a new knitter this knowledge is still missing. Thanks ❤

Metropolis Cardigan by The Fibre Company (Norah Gaughan)

from Instagram: