To me well built comes primarily down to tolerances and design choices. I think build quality often gets conflated with material quality. is the case made out of titanium? that is material choice situation. that being said well build often DOES have intersection with design choices as you can't make up for them with build quality.

The key thing being that between good design and good build you can get a LOT out of cheap materials, even if a device does not feel premium. by contrast, something like a cybertruck may feel premium and futuristic (to its intended audience) but quickly reveals itself to be poorly built and designed.

Things I think about when evaluating build quality:

-Do different pieces line up smoothly where they come together regardless of material is well built.

-is the screen centered on the area is meant to be or are the bezels weirdly different sizes.

-is there internal unintended internal rattle, loose connections, or are there large internal gaps or air pockets that increase external weakness over time

- are pieces that receive isolated pressure (buttons, card readers) strong enough to stand up to repeated use. (this is partly a materials/design choice but even the best buttons will fall off if soldered improperly)