What do keyboards, message filtering, call directory, content blockers and web extensions, ReplayKit and network extensions each do, and what constrains them?

Beyond sharing, iOS has extension points that plug your code into system features. Each has a narrow job, a privacy boundary and often an entitlement. Custom keyboard (UIInputViewController): replaces the system keyboard; it is sandboxed away from the network and the shared container unless the user grants Full Access (RequestsOpenAccess), never sees secure text fields (passwords, which always use the system keyboard) and has a very tight memory budget and no access to the microphone by default; keep it light and show a way to switch keyboards. Message Filter (ILMessageFilterExtension): classifies unknown-sender SMS as junk or promotion or transaction, either offline from the content or by a network lookup to your server, and the privacy rule is that the system sends only the sender and text of unknown senders, with no reply path. Call Directory (CXCallDirectoryProvider): supplies phone numbers to block or identify; you add numbers in ascending order in bulk (not per call) from a list the app prepared in the shared container, and the system reloads it; there is no real-time lookup of an incoming call. Unwanted Communication reporting offers a way to report junk. Safari Content Blocker: you give Safari a JSON rule list ([{"trigger": {...}, "action": {...}}]) and Safari applies it without ever sending browsing data to the extension, which is why it is considered private; Safari Web Extensions reuse WebExtensions (JavaScript, manifest) with a native handler and need user permission per site. Broadcast Upload and Broadcast Setup (ReplayKit): receive the screen frames and audio of a live broadcast in a separate process with a 50 MB budget, so you must stream the buffers to a server without keeping them.

Network extensions (NetworkExtension) are the most powerful and the most restricted: a packet tunnel provider (a VPN), an app proxy, a DNS proxy or DNS settings, a content filter (filtering traffic in a managed device), and transparent proxy. They need a specific entitlement and capability that Apple grants on request, run in their own long-lived process with a memory limit and sit in the network path, so they are subject to strict review, user permission (the VPN configuration prompt) and heavy testing. AuthenticationServices extensions (credential providers for passwords and passkeys, and the single sign-on extension for enterprise) integrate with AutoFill and managed devices. Choose the right point by the system feature you extend, not by convenience, and remember the shared rules: cold start, small memory, no UI of your own beyond what the point allows, shared data through app groups and the Keychain, and review criteria that vary by point (keyboards and network extensions get the closest scrutiny). Treat entitlement-gated points as a project, not a task: request the entitlement early with a clear justification, since the answer can take days and may be a refusal.