The arc is the sum of the beats, not a separate thing to design. Plot the tension of each beat on a line and the arc is what you see: a good one rises unevenly, with releases that are genuine and brief, and a final rise that is steeper than any before it. A flat line with a spike at the end is the most common shape in corporate video and the reason so much of it is watched at double speed.
The useful diagnostic is not "does it have an arc" but "where does the line go down and why". Every descent is either a deliberate release, which is fine and necessary, or a beat doing no work, which is a cut. There is no third case.
Arcs also exist at other scales. A sequence has one, a scene has one, and a whole campaign has one. The most common structural problem in a series is that every episode arcs beautifully and the series does not arc at all.
What is a narrative arc?
The shape of rising and falling tension across a story. It is produced by the beats rather than designed separately, and it is diagnosed by looking at where the tension falls and asking whether each fall is deliberate.
How do you fix a flat narrative arc?
Find the beats where nothing changes and cut them, then add a real reversal at the centre. Adding more incident rarely helps; a flat arc is usually a change problem rather than an event problem.