My understanding of a "distro" is as being a curated collection of components from lots of different sources which have been re-compiled and tested to work together. At its core is always a specific version of Linux and then the userland packages that users may choose to install.

The most fundamental alternative would be to build the core Linux system, e.g. following the Linux from Scratch process and then downloading each application from its repository and compiling, installing and configuring. That too is a valid choice but one that some of us don't care to indulge in.