Push notifications, WebSockets, and SSE: which is best for real-time updates?

hey folks im architecting a new feature that requires real-time notifications to users we're expecting potentially millions of active users im weighing the pros and cons of different approaches like traditional push notifications via APNS or FCM WebSockets and Server-Sent Events SSE each has its own scaling challenges and client-side complexities what are your experiences with scaling these for a massive user base any specific patterns or libraries youve found essential for managing connections and ensuring reliable delivery at scale