I have been through a lot of websites which have the following kind of demonstration images/icons which look plain but awesome. What are they exactly called? What is the term for such graphics?
Any good tutorial for learning or a link to start with would be appreciated.
The term(s) you are looking for is/are "Mockup" or "wireframe" or "UI Kit". There are a lot of free vectors available for UI mockups (by device type).
Searching the Adobe Creative Cloud App's Assets tab for "UI" turned up a lot of these.
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.
Examples:
https://dribbble.com/search?q=flat+design
but awesome
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.
