Almost all of it is workmanship and layout planning. Slab size sets how many seams you need, but where they land and how they're finished is a decision someone makes. Ask to approve the seam placement on a layout drawing before fabrication, and ask whether they template digitally, because that's where tight joints come from. A good seam is visible if you look but shouldn't be something you feel. We had
Quartz Countertops in Charleston, South Carolina installed by Perfect Stones and they walked us through the layout first. Also, pattern matching across a seam costs a bit of extra material, so ask whether that's included.