The Crew 2

You unlock cutscenes/narrative pieces by starting new disciplines for the first time as well as completing certain races. You can check your progress via Pause Menu (Profile tab) > Pilot > Narrative.

Just to be more specific than other guides, I found that these events are the only ones that count towards the achievement, so just complete all the following in any given family:

STREET RACING FAMILY
- Street: The Sleeper, The Accident
- Drift: The Charity Event
- Drag Race: Tweak and Shout
- Hypercar: Latrell’s Car Part 1, Latrell’s Car Part 2
- Boss: Beat Tio Marquez

OFF-ROAD FAMILY
- Rally Raid: High Speed Drifter, Portfoliage
- Motocross: Wolf Territory, Downhill from Here
- Rally Cross: American Petrol Race
- Boss: Beat Tucker

FREESTYLE FAMILY
- Aerobatics: Push it to the Limit
- Jetsprint: Las Vegas Waterway, Go Against the Flow
- Monster Truck: Wreck and Havoc
- Boss: Beat Sofia

PRO RACING FAMILY
- Powerboat: Über Dam, Not so Quiet on the Eastern Front
- Touring Car: Going Back to My Roots
- Air Race: The Delivery, Viva Flight Vegas
- Boss: Beat Clarence

Once all the events and boss of a particular family are done, head to the 8th narrative piece of that family and press play. Once the full video clip ends you should unlock the achievement.

NOTE: Although I got all cutscenes for all families during my playthrough, I unlocked the achievement after only watching ONE of the families full narrative piece (Street Racing in my case). If the achievement doesn't pop after watching one full video clip, you can try watching it again or you may need to watch another one or all 4 of them (one for each family).

Long guide, sorry! There are 8 narrative pieces per each main event (Street Racing, Offroad, Freestyle, and Pro Racing). The narrative pieces are automatic story cutscenes from playing the story races. They are not actual gameplay cutscenes, but more artistic stories surrounding the main event you are doing. You can tell which races will have a cutscene based on the artwork of the race when selecting an event. Here is an example below.

The race that I have outlined in red has the same color schemes as the other races, but has a different art work when compared to the normal races with the skull artwork. (Keep in mind, there are some races like the papa johns Street race and the Red Bull drift events that have other art works, but these aren’t actual story mission, more just sponsored races, so those don’t count). This means that in each main event, there are 7 unique races spread across the four disciplines in each specific main event.

General info and tips!
This achievement is just tied to completing those 7 specific races, not tied to a certain percentag of an entire main event or anything like that.

After getting the 7 pieces you must view the 8th piece in the Narrative menu. After all have been played, achievement unlocked.
If anybody is still confused, comment your question and I will try to answer it in the best possible way.