The style doesn't necessarily have a name; they're simply illustrations with a limited color palette.
Where do they gather these kind of gradient colors that fit so well?
How does the designer know where to place those shadows and lights?
You have to learn how to represent light and shadow. There are countless books and tutorials specifically on this subject, but you can start by look at objects and scenes around you.
See how light goes through the arches and columns in this photo:
In this other photo, look how light and shadow create an almost monochromatic scene where the different tones help you perceive the depth:
If you want to be an illustrator, learn how light, shadow and depth behave in the physical world first so you can transfer that to a 2D medium.