In the normal mode of edition, when you add a note, it will eat up rests/notes to make room for the newly added note. In the same way, if you remove a note, rests will be added to fill the space that was taken by the note.
When the insert mode is activated, adding a note will instead push the content to the right. Removing a note/rest will pull back the content.
While you are in insert mode, clicking on a note duration/dots will no longer affect the note at the cursor position. It will instead select the duration of the notes to be inserted.
In the same way, you can choose the accidental to apply to the note to be inserted:
If you are editing an unpitched part, you can choose the notehead to apply to the note to be inserted:
When using the insert mode, you can also insert extra rests at the location of your cursor.