Looks? I build giant boxes and put giant automation in them. I can’t be bothered to make giant box look nice when I got my factory to automate!
For me form comes before function quite often in that I’ll build the contraptions first, but I’ll build them with a build or exterior in mind so by the end it’s a good balance.
I’ll usually try to do both, but if it’s not possible, function has higher priority.
A bit of both.
I don’t typically build things just for the looks, but once I’m building them I take aesthetics into account. Just the usual - picking a style that fits what I’m building (e.g. cottagecore for villager-powered farms, industrial for Thermal mods, steampunk for Create etc.), design the thing in creative mode, then build something that fits the terrain OK to host my contraption.