For a writer, an ensemble cast is a challenge (and a whole lot of fun). Depending on the genre, the characters can be quick, eccentric sketches or fully developed. Ensembles are no easy feat. You don't want an upstart in the ensemble to usurp the protagonist or the antagonist. They serve as mirrors. A protagonist surrounded by fully realized characters who offer another view of your hero (or villain), actually make primary characters shine, bring out the best (or worst) in them, reflect strengths and weaknesses.
A collection of characters shouldn't be a challenge to the reader, but a fabulous discovery. An ensemble is a breeding ground for amusing or heartbreaking conflict, shattering betrayals, astonishing reversals, secrets, love, hate. While the reader must be invested in your main character (so make them interesting, the one who experiences most of the above), an even divide of attention for the cast is acceptable, as long as there aren't too many people. Each cast member should have a story arc that gracefully syncs with the protagonist's.
Back stories are fun ways to keep characters interesting. Even though you probably won't use half of what you invent for your ensemble, a back story creates a unique person and can even trigger key elements in your plot. I keep a journal for every book I write, and fill it with place names, turns of phrase, sketches, character stories, letters, biographies, etc;
For fantasy writers, ensembles turn us into ringmasters, because we really need to keep the three-ring acts moving and not let anyone outshine the main character. Tolkien's Lord of the Rings is a classic combo of characters, all with grudges against each other and back stories that are legendary because they're all connected to legends. Each of Leigh Bardugo's Six of Crows criminal gang has a story that explains how they've been damaged in some fashion at such young ages. And Game of Thrones has kings and queens, upstarts and commoners, all battling one another for one thing.
There are other ensemble tropes: A group of friends are confronted by evil, usually in the form of a monster. Stephen King's collection of friends in It, for instance, grow up with psychological trauma caused by the monster and their own childhoods. The Pevensey family from C.S. Lewis's Chronicles of Narnia have the horrors of World War II to deal with before they step into a fantasy world where they each follow their own path. The Narnia books can also be considered a family ensemble. This trope plays out in mystery stories and in horror, as well, when a family has to deal with secrets and betrayals that will end them or make them stronger.
Combining ensemble tropes is popular in fantasy. A murder mystery and a fantasy? Jay Kristoff's NeverNight, set at a school for young assassins, or Seanan Mcguire's Every Heart a Doorway, which takes place at an academy for young people who've been to other worlds, are fantastic examples of mixing ensemble tropes. Horror fantasy? Stephen King's Dark Tower series has horror elements surrounding a fabulous cast on a fantastical quest. And Barbara Hambly's Time of the Dark series has two people finding themselves in a different world, surrounded by a cast of characters at war with one another and Alien-like horrors.
What does an ensemble cast offer?
1) Diverse personalities to bring out unique elements in your main character or antagonist
2) Individual character arcs that help drive plot
3) Complications/help sources for the protagonist
4) More characters to root for. (Or fear).
5) A bounty of secrets, betrayals, love, humor.
6) Other characters who see the protagonist in different ways and help to make the hero more three-dimensional.
Ensembles are a fantastic way to create a complex world for your main character, keeping you from stereotypes and cardboard settings.
