Bootstrap/Foundation can be great when you don't know a lot and need something that is acceptable now. However, from a gaining experience point of view, those frameworks can get kind of complicated and trying to figure out how some effect is being achieved can be very difficult. I've found there's a certain level of skill you have to get to before you can actually use those frameworks to learn new techniques. When you get to that point, they can be great vehicles for learning how to do certain things.

I'm currently using foundation with SASS[0], Compass[1], and Susy[2]. I only take certain parts from Foundation as I need them. Foundation is built on top of SASS so it's easy to pull out parts of the framework. I can say that using a css preprocessor (SASS/LESS[3]) has really made a big difference in the speed I develop and the maintainability of my css. To directly answer your question:

>>In other words, how would an expert go about building a responsive beautiful website like, https://www.lily.camera/

They probably use a preprocessor like SASS or LESS and either build it from scratch or take parts from different frameworks that they need and mix and match. Looking at their html source, they're not using a framework directly, but they could still be using one with their css preprocessor which is compiling down to their production css. Also, there's a good bit of javascript going on in that site as well. Hope this helps.

[0] http://sass-lang.com/ [1] http://compass-style.org/ [2] http://susy.oddbird.net/ [3] http://lesscss.org/