Linux is a very powerful and complex operating system kernel that can be married to various types of user space, ranging from a simple embedded device, through increasingly complex mobile devices using Android, to a full server operating system. One of its strengths is the degree of configurability. The definitive place to get the source code is https://www.kernel.org/, but you will probably need to get the source for a particular SoC or board from the vendor of that device or a third-party that supports that device. The customization of the kernel for a particular target may consist of changes to the core kernel code, additional drivers for devices that are not in mainline Linux, a default kernel configuration file, and a device tree source file.
Normally, you start with the default configuration for your target board, and then tweak it by running one of the configuration...
