Short answer: you can do everything (that you can do in C) in Nim.
Longer answer:
"Is it suitable for low-level programming" - depends on what you mean by that, but yeah, you can go as low-level as creating your own kernels in Nim. You can use Nim on microcontrollers too.
"What is created with its help" - everything can be created, from simple terminal applications to complex web aplications with both backend/frontend in Nim.
"Is it suitable for writing malware" - yes, e.g. see https://github.com/guitmz/nim-cephei
"Networks, web, gui" - there's a lot of libraries, but some of the most popular: https://github.com/dom96/jester https://github.com/trustable-code/NiGui https://github.com/StefanSalewski/gintro
You can check talks from NimConf 2020 - they really represent a lot of different places where you can use Nim :) https://www.youtube.com/playlist?list=PLxLdEZg8DRwTIEzUpfaIcBqhsj09mLWHx