In the ending of The Fate of the Furious, Dom turns on Cipher, destroys the submarine by turning her homing missile against it, and rejoins his crew once Deckard Shaw returns his son. The turn happens during the fight on her plane, where Dom kills her enforcer Rhodes. Cipher escapes by parachute, so she remains a threat.
Here is the finale broken into its steps, the way a mechanic would read a job. First, the team stops the sub's weapons and blocks its way out across the frozen sea. Second, Deckard and his brother Owen reach Cipher's plane to get Dom's son out. Third, Dom switches sides mid fight and kills Rhodes, the same enforcer who killed Elena Neves in front of him in New York. Fourth, Cipher fires a homing missile at Dom's Charger. Dom does not try to outrun it. He lures it into the submarine, and the submarine is destroyed. The missile Cipher built to finish him becomes the tool that ends her plan.
The frozen sea has its own production story. Screen Global Production reports that the lake finale was shot in northern Iceland, standing in for the Barents Sea, and that the ice had to be thick enough to hold nearly 20 vehicles at speed. That points to real vehicles on a real frozen surface, though no production account separates the practical shots from the digital ones. The same report says Alaska, northern Canada and Argentina were considered first, but El Niño conditions in 2016 made North American lakes unsuitable.
The film's earlier set pieces set up this payoff. The Havana street race that opens the story is a one on one contest, and the New York chase past the Flatiron Building and through Times Square is a heist on wheels. The submarine finale is the first time in the film that the whole crew, plus Hobbs and both Shaw brothers, works toward one goal at one place. Opinion: that structure makes the last act feel like a reunion before the film says so out loud.
The series context helps here. The Fate of the Furious is eighth in both release order and chronological order, unlike Tokyo Drift, which was third in release order but sixth in chronological order. It follows Furious 7, and its ending carries that film's thread forward: Deckard Shaw, an antagonist before, is accepted into the crew after handing Dom's baby son back. The next release, Hobbs and Shaw, builds a spin-off on the pairing of Hobbs and Deckard, who share the prison break scene here.
The closing beats are quieter. Hobbs turns down a government job to spend time with his daughter. Dom names the boy Brian, and the group shares a rooftop dinner that closes the film on the team being whole again. There is no post credit scene, per TheWrap, so the dinner is the last image.
The Fast and Furious timeline maps how this film leads into the later entries, and the upcoming film is covered at Fast Forever.