Short answers, pulled from the story.
Lawrence Kasdan insisted on creating a renegade stormtrooper named Finn during script meetings for The Force Awakens. J.J. Abrams directed the film and selected John Boyega to portray this new character.
Finn's designation FN-2187 references Princess Leia's cell number from the original 1977 Star Wars film. It also nods to Arthur Lipsett's 1963 short film 21-87.
Finn survives all events in Star Wars The Rise of Skywalker without dying. He leads a surface attack against Sith Eternal forces on Exegol after becoming co-general following Leia's death.
John Boyega had impressed director J.J. Abrams in Attack the Block released in 2011 before being offered the role. Kathleen Kennedy called Boyega a top candidate from the start of casting.
Finn fled the galactic conflict entirely after feeling sickened by the cruelty he witnessed within the First Order. He returned to help disable Starkiller Base's protective shield when the First Order destroyed several planets and took Rey prisoner.