Question about clFFT (OpenCL)

I have installed succesfully clFFT library (found it on github) of OpenCL.

I observe that in the clFFT directory source code, there is not any .cl files or kernels.
It consists mainly of .h, .c and .cpp files.

My question is how is it possible, without having any .cl files to contain code that is executed on GPU via Opencl?

GPU code in OpenCL doesn’t have to be in a .cl file. It can be in other files such as in .h or .cpp files also.

Here is an example:

https://www.olcf.ornl.gov/tutorials/opencl-vector-addition/