Push in one paragraph
Remote push works through a delivery service: the app registers and receives a device token, your server sends a message to the service, and the service delivers it to the device. On Android that service is Firebase Cloud Messaging (FCM). On iOS it is APNs. Everything else is a client library or a product built on top of those two.
Firebase Cloud Messaging (FCM)
FCM is the standard foundation. It handles Android natively and routes to APNs for iOS. You control everything: the project, the tokens, the sending. The cost is setup: a Firebase project, the google-services config, and (on iOS) the APNs key wiring.
It is the right choice when you want full control and are comfortable owning the plumbing.
OneSignal
OneSignal is a fuller product on top of the same delivery: a dashboard, segmentation, campaigns, and message journeys you can run without a developer. You trade some control for convenience.
The honest caveat: it is a third-party service between you and your users, with its own pricing tiers. And on iOS it requires a native notification-service extension for full functionality, a detail that matters when your app is built by a tool that does not add native code.
The decision, honestly
There is no universal winner. FCM if you want ownership of delivery. OneSignal if you want a product, not plumbing. Both run on your own accounts either way.
What should never be negotiable is whose credentials are used. If a builder insists on using its own Firebase or its own push account for your app's notifications, your users' tokens and your sending identity belong to someone else. Tapbound's push add-on is the opposite: you configure your own Firebase or OneSignal project in your dashboard, and your app registers against it. Your tokens, your account, your data, managed on your end.