There are two modes in the timeline: Overwrite mode and Insert mode. You may click the "Insert/Overwrite" icon to switch between the two modes.
Adding files to timeline under different tracks will lead to different results.
Under the Insert mode, a new file can be added to a track on a selected location without overwriting the existing files in the track.
For example, as shown in the screenshot below, there are already two clips on a track. If you add another clip to the track between the existing clips under the insert mode, the file will be added with no influence on the existing clips.
Under the Insert mode, a new file can be added to a track overwriting the existing files on the track.
As shown in the screenshot below, if you add a new clip between the existing two clips under the Overwrite mode, the file on the right side of the blue cursor indicator will be overwritten by the newly added clip.