which look plain but awesome
The 'look plain' part is probably the current trend of geometric, flat design. A response to the more highly stylized trend of the past decade in UI design.
That's harder to define, but, in a nutshell, these are just well drawn adhering to standard principles of good design (color theory, balance, contrast, proportion, scale, etc...)
There's no one particular tutorial to explain any of that, but in general, you'd use software such as Adobe Illustrator or Inkscape for a lot of this type of work.