Smooth looking walls aren't rocket science - you just require smooth walls! You need a smooth skim coat of plaster over the drywall done by someone who knows what they're doing, then paint applied without adding texture. Smoothest finish is to spray paint the walls without any rolling, but this is generally considered a bad idea because it won't hide any imperfections at all and it will be impossible to do touchups. For our house we had the paint sprayed then backrolled with a fine nap roller (maybe 3/16ths inch?). It looks gorgeous, but it is indeed quite difficult to hide any patching. It's another example of the minimalist contradiction: less texture requires a lot more work!

Yes indeed! More work is required for a cleaner look. The interior GWB walls in this residence received a skim coated (Level 5) finish.

what color white did you use for the walls? Thank you!

Walls are painted Benjamin Moore Atrium White. Thanks for the inquiry.

What color red did you paint the door? It is bright and elegant, just the right amount of loudness.