This document details the Chromium browser's Layer 2 architecture, which is responsible for coordinating the structure and navigation of complex webpages across multiple renderer processes. The core object is the FrameTree, which maintains the hierarchy of frames on the browser side, mirrored by individual FrameTreeNodes that hold persistent frame identity and state. Navigations are controlled by the Navigator, which handles incoming URL requests, determines necessary security isolation (SiteInstance), and orchestrates the loading process. Crucially, the RenderFrameHostManager is attached to each frame node, managing the complex logic of process switching, creating new RenderFrameHost instances for cross-site content, and setting up proxies to ensure secure communication between frames in separate processes. This layered infrastructure enables features like Out-of-Process iframes (OOPIFs) and multi-page architecture (MPArch), ensuring secure and reliable content management during frame creation, navigation, and cleanup.