The action of adding elements is not reserved for the timeline. It is also possible to use the canvas for this purpose, but if you have already used other video editing softwares, you will probably be used to add elements by dragging to the timeline. Adding elements is no more secret.
All the files you have imported are at disposal on the Media library, as well as all interactivities and complements (Addons). For each element you would like to add to the project, click on it on the library and drag it to the timeline or to the canvas.
There is a difference between dragging to canvas and dragging to timeline. By dragging an element to timeline, you will automatically place it in the middle of the visible part of the timeline. However, if you drop it on the canvas, you will be placing it on the current point of the timeline cursor (red line).