Apple's Foundation Models can now use third-party LLMs (Claude, Gemini) [video] (developer.apple.com)

Text summary since it's a video: it's a model abstraction layer. You write against a LanguageModelSession, import a Swift package, initialize the model you want, and pass it to the session. Apple ships SystemLanguageModel (on-device), PrivateCloudComputeLanguageModel, and open-sourced CoreAILanguageModel and MLXLanguageModel for local models; Anthropic and Google are publishing their own packages.