It sounds like you're using the New Rich Content Editor (RCE). The old RCE had a Content Selector that stayed on the right and was convenient for adding course content because it stayed there.
In the New RCE, adding content is done by selecting one of the four menu options from the toolbar and it is a much harder process. Canvas recognizes this (RCE Updates - Summer 2020) and is working to improve it before it is forced onto people.
For example, if you want to add a link to course content, you click on the Link dropdown and then choose Course Links.
The four buttons from left to right are Links, Images, Media, and Documents (previously called Files).
Additional help can be found in the New Rich Content Editor section of the Canvas Instructor Guide.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.