Where to Look

A quick index for “I want to change X → start in file Y”. All paths are under OpenSim/.

I want to change…	Start here
Region startup / bootstrap	`Region/Application/OpenSimBase.cs`, `Region/Application/OpenSim.cs`
How a region is loaded from disk	`ApplicationPlugins/LoadRegions/` (`LoadRegionsPlugin.cs`, `RegionLoaderFileSystem.cs`)
First-run interactive config	`Framework/ConfigurationMember.cs`
Merged INI config behaviour	`Region/Application/ConfigurationLoader.cs`; `.ini` in `bin/config-include/`
A viewer command / packet	`Region/ClientStack/Linden/`; contract in `Framework/IClientAPI.cs`
Avatar / agent behaviour	`Region/Framework/Scenes/ScenePresence.cs`; `Region/CoreModules/Avatar/`
Prim / object behaviour	`Region/Framework/Scenes/SceneObjectGroup.cs`, `SceneObjectPart.cs`; `Region/CoreModules/World/Objects/`
Terrain	`Region/CoreModules/World/Terrain/`
Parcels / land / estate	`Region/CoreModules/World/Land/`, `Estate/`
Permissions	`Region/Framework/Scenes/Scene.Permissions.cs`
A grid service (asset/grid/inventory/…)	`Services/<Name>Service/` + its contract in `Services/Interfaces/I<Name>Service.cs`
Talking to a remote service	`Services/Connectors/`
Wire messages (sim ↔ sim, sim ↔ Robust, HG)	Protocols – request/response bodies, not the C#
Login	`Services/LLLoginService/LLLoginService.cs`; HTTP entry in `Server/Handlers/Login/`
Teleport / region crossing	`Region/CoreModules/Framework/EntityTransfer/EntityTransferModule.cs`
Hypergrid travel	`Services/HypergridService/`, `Region/CoreModules/Framework/EntityTransfer/HGEntityTransferModule.cs`
Physics	`Region/PhysicsModules/` (`BulletS`, `ubOde`, `POS`, `BasicPhysics`, `SharedBase`)
Scripting (LSL/XMR)	`Region/ScriptEngine/` (`YEngine/`, `Shared/`)
OAR / IAR import-export	`Region/CoreModules/World/Archiver/`
Asset cache (Flotsam)	`Region/CoreModules/Asset/FlotsamAssetCache.cs`; `config-include/FlotsamCache.ini`
Adding / editing a region module	Define interface in `Region/Framework/Interfaces/`, implement in `Region/CoreModules/` following `IRegionModuleBase`
Writing an optional region module	`Region/OptionalModules/` (examples in `Example/BareBones*`); hook `scene.EventManager` — see Region module events
Scene events a module can subscribe to	`Region/Framework/Scenes/EventManager.cs`
Per-client (viewer packet) events	`Framework/IClientAPI.cs`; subscribe from `EventManager.OnNewClient`
The Robust server itself	`Server/ServerMain.cs`, `Server/Base/`, `Server/Handlers/`
Database layer / a new store	`Data/` (`SQLite`, `MySQL`, `PGSQL`, `Null`) + `Data/Base`
The console / commands	`Framework/MainConsole.cs`, `Framework/ICommandConsole.cs`; commands registered via `ICommandConsole`
Auto-backup	`Region/OptionalModules/World/AutoBackup/AutoBackupModule.cs`
Groups / Offline IM / Voice add-ons	`Addons/`

Patterns to know

Discovery over new. Features are wired via Mono.Addins extension points ([Extension(...)], [TypeExtensionPoint(...)]), not hardcoded instantiation. Search for the interface name to find implementations.
Module interfaces as the API. To use another subsystem, call scene.RequestModuleInterface<T>(); to provide one, call scene.RegisterModuleInterface<T>(this). Same idea process-wide via OpenSimBase.ApplicationRegistry (IRegistryCore).
ReplaceableInterface. A module declaring ReplaceableInterface = typeof(IFoo) can be swapped by another implementation of IFoo; only one loads. Use this when adding an alternative implementation of an existing feature.
Services/Interfaces vs Connectors. If you change a grid service’s behaviour, edit the implementation in Services/<Name>Service/; if you change how the simulator talks to it, edit Services/Connectors/ or the *ServiceConnectorsOut module.
Partial Scene. Scene is split across many files by concern (Scene.Inventory.cs, etc.). Edit the file matching the concern.