>>2684havent seen the movie but have played sekai a bit
basically miku encourages teenagers to get in touch with their emotions and work through the trauma or whatever they have. kind of acting as a mentor and a therapist
for example, leo/need (the one with the blue long haired girl and the blonde/pink haired chick) has miku help a group of childhood friends reunite after misunderstandings caused them to split apart
every group and character has their own kind of problem and you can pretty much just read the wiki for them. the sekais are worlds for the OCs to just chill out if they need to and talk to miku, they kind of represent all of the members' mental states, memories, and goals i guess
all of this is from memory, i played the game when EN early access launched and not much more so that's all i have to say. read the wiki and you'll be ok, you don't even have to learn about all of the units just pick one with characters that appeal to you and ignore the other ones