I was having trouble inserting an empty clip in the main timeline to be able to add more footage after the b-roll footage ended.
insert gap. Its the same as a slug in older versions
If I understand what you want, put the playhead at the end of the connected clip and press D to do an overwrite edit.
To insert a gap clip: Choose Edit > Insert Gap (or press Option-W).
To insert a placeholder clip: Choose Edit > Insert Placeholder (or press Command-Option-W).
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