UnrealBuildTool (UBT) is a custom tool that manages the process of building Unreal Engine 4 (UE4) source code across a variety of build configurations. Read BuildConfiguration.cs to explore various user-configurable build options.
Modular Architecture
UE4 is split into many modules. Each module has a .build.cs file that controls how it is built, including options for defining module dependencies, additional libraries, include paths, etc. By default, these modules are compiled into DLLs and loaded by a single executable. You can choose to build a monolithic executable in the BuildConfiguration.cs file.
It is important to understand that the build process executes independently of any project files for the development environment, such as .sln or .vcproj files (for Visual Studio). These files are useful to have for editing purposes though, so there is a tool being provided to generate them dynamically (based on the contents of your project directory tree). You can run this tool with the GenerateProject.bat file located in your [Root UE4] directory.
Note: Running GenerateProject.bat from time-to-time will keep your code editor up-to-date with files that are being added to (or being removed) from disk.