Hi @Pam !
I will post links to some portfolios of great concept artists. Notice that they were successful professionals but a lot of their artwork seems very sketchy and rough, but that is the idea of concept art: you are creating a concept, the starting idea of the visual look/mood of a character, a building, a landscape, a scene. Some of them have a mixture of sketchy and more polished work, but the basic principle is that you must be able to create several ideas of a character or building in one go, like fill an entire page with ideas, thumbnails, sketches and so on. Then re-work some of the ideas and polish them. Then it is presented to the other artist in the company for him to finish the work. Like, give your concept to the 3D artist such that she/he can create the 3D model (and even modify a bit).
Also, notice that it is a very specialized profession that exists due to the high demand in the industry to quickly create games and movies, so being capable to mass produce in a day is a must. It's not like illustration that we can dedicate 1, 2 or even 3 days to a single full piece.
Kingston Chan http://kingstonart.blogspot.sg/
Machinarium Concept Art (Game): http://selmiak.bplaced.net/games/pc/index.php?lang=eng&game=machinarium&page=concept
Xa-xa-xa (Wakfu game concept artist): https://xa-xa-xa.deviantart.com/art/croquis-2-78332818
Simon Kopp (concept art for the game "Ori and the blind forest"): https://www.artstation.com/acapulco
That being said, here are a couple of videos of professional artists talking about their views on the Concept Art career:
And an interesting perspective on Adam Adamowicz approach to concept art: https://kotaku.com/the-life-and-creativity-of-a-great-bethesda-artist-1740993491