@AndrewB1 ...
In addition to the information that you got from @gnoack, another thing that you can do is try out the course import process in your school's "test" environment. (This way, you can see exactly what happens when importing content ... if it overwrites, replaces, adds to, etc.). This "test" environment is completely separate from your school's "production" environment ... which is the environment that you typically sign into on a day to day basis. Testing all this out in your "test" environment will not disturb anything that you've done in your "production" environment ... as this environment is meant for you to do any kind of testing you want before doing things in "production". I always tell people to make sure you sign out of "test" before returning to "production" ... so as to not get confused about which environment you are in. Here are some Guides for you on the "test" environment:
Hopefully these Guides will be of help to you. Sing out if you have any other questions.
