Guest Guides for the additional story content developed after Chapter 3's release. As of patch 3.8.0, the Brotherhood of Steel, Institute and Railroad Remnant questlines, the Magnusson epilogue and Ashes to Ashes, Rust to Rust are part of Sim Settlements 2 - Chapter 3, so they no longer require SS2 Extended.
The quest guide pages linked below will contain spoilers.
Requirements: Brotherhood of Steel is destroyed and State of War is in progress, or Chapter 3 is completed.
Requirements: Institute is destroyed, State of War is in progress, or Chapter 3 is completed.
Requirement: Epilogue: Thanks for the Memories is completed.
Requirements: Railroad is destroyed and State of War is in progress, or Chapter 3 is completed.