The process is easiest when the course you want to add content from is in your Canvas and you have access to that course. What is the Course Import Tool?
Settings > Import Course Content > Select Specific Content
And it's especially nifty if all small portion that you want is already organized in a module, so you can just import that module and all the pages, assignments, discussions, quizzes, and related files come with it.
Cheers - Shar