The Bumblebee movie (2018) ends with Charlie letting Bumblebee go, Bumblebee taking a new car form, and a reunion with Optimus Prime that sets up the Autobots' arrival on Earth.
After the battle, Bumblebee and Charlie say a tearful goodbye on a cliff overlooking the Golden Gate Bridge. He turns into a 1977 Camaro and drives away to carry on his mission, while Charlie returns to her family and Memo and finishes repairing the old car she once worked on with her father. Bumblebee then reunites with Optimus Prime, who survived, and they watch more Autobot escape pods head toward Earth.
Here is my reading, and it is opinion. The Camaro swap is the story's quiet payoff. The Beetle was the shell Charlie bonded with, the car her uncle handed her and the one she woke with a signal that pulled Shatter and Dropkick to Earth, so a new body means Bumblebee has to leave that bond behind, and the film lets that cost something. Charlie's finished car mirrors it, since both of them end the film with something repaired. She spent the film grieving her father, and the last shot of her is not a rescue but a repair she completes herself.
The ending also answers a question the earlier films left open. In Transformers (2007), Sam Witwicky buys a beaten up car that turns out to be Bumblebee, and the film treats the bond as a given. Here we see the bond being built, scene by scene: the electric jolt that restores his memories, the voice Blitzwing stole, the neighbor Memo keeping the secret. In my opinion, that is why the goodbye lands harder than the reunions in Michael Bay's five films, where the Autobots tend to arrive already loyal to a human.
The Optimus scene is the franchise handshake. Peter Cullen voices him, as he does in the 2007 film, so a scout who lost his voice in 1987 is tied back to the larger war in one cut. It is also a quieter closing beat than the large scale finales of Transformers: Age of Extinction and The Last Knight, both of which run over 150 minutes. Travis Knight keeps this one to 114 minutes, with Steven Spielberg as executive producer and Michael Bay credited as a producer, and the finale at the harbor tower is staged around Mare Island, a former naval yard, according to Screen Global Production. Dario Marianelli's score, the first in the live action films not by Steve Jablonsky, scores the farewell as a goodbye between friends rather than a victory. For where the escape pods lead, the transformers timeline lays out the order.