Deus Ex is what is called “mechanical design” where parting lines are slightly exaggerated. We add more parting lines to give the effect that there’s more parts to the object. It would be an engineering’s nightmare to try to produce those objects! 
We also base on current objects or prototypes to design our props. We avoid doing to much “inventions” because the game is NOT Sci-fi, it’s near future.
Good question! It was an easy switch. I knew how to draw well and had good technically knowledge of how objects are built. Learned all that in design firms. De-molding angles, where to put screws, using different materials, etc. I was always a curious guy and always took things apart. So, when I had my first gig in 2000 for a movie to design car interiors, I was confident that I could do the job. It was a design field (transportation) that I wanted to go into. It’s amazing that you get payed to draw cool things without too many design limits. After my first movie, I was asked to do props design for others. No problem, it’s industrial design to me.
However, doing just props design does limit you to specific movies. Movies that require cool sci-fi designs. If I drew environments and characters, I would have been busy all the time. Same for the gaming industry. It all depends of the subject. Most game companies ask to be versatile (environment and character) concept artist. If there’s not a lot of props to design, characters or environment concept artist will do it. In my case, I’m lucky to have and is working on a game that requires very specific props design (including weapons).