Glass Onion: A Knives Out Mystery is a Netflix Original film, meaning it is exclusively available for streaming on Netflix. You will need an active Netflix subscription to watch it. As a Netflix production, it is not typically available for rent or purchase on other digital platforms like Apple TV, Google Play, or Amazon Prime Video, nor is it currently in cinemas.
This exclusivity is part of Netflix's strategy to provide unique content to its subscribers, so if you want to watch the continuing adventures of Detective Benoit Blanc, Netflix is the only place to find it.
Pro tip: If you're looking for other films in the mystery genre, Netflix has a strong catalog, and often highlights similar titles once you've watched a popular movie like Glass Onion.