first you need to create a folder on the Orion server for NTA reports,
C:\Program Files (x86)\SolarWinds\Orion\Reports
Then go back to the Orion web - login with Admin >> edit the Accounts on Manage Account >> set the folder permission >> drop down and put it on the folder you created.
Then give them access to the tab for Report >> edit their menu bar and put reports.
So if I do the above, will the user have access to NTA database and NTA reports only?
I mean they won't be able to access NPM database, sew NPM CBQoS kpis, or create reports (for example router utilization which is usually comes from NPM not NTA )
yes...If you will access that path - it is the general folder for reports. Creating a folder on that folder limiting the user that he/she can access only the content on which folder he/she has assign to. Also he/she can also just save on that assigned folder.