Qt libraries and headers are packaged with the Maya devkit for convenience.
The Qt libraries are located in devkitBase\devkit\lib on macOS and Linux. On Windows, the link libraries are located in devkitBase/lib, and the DLLs are located in devkitBase\devkit\bin.
The Qt header files are packaged as a zip file in the devkitBase\include directory on Windows, and as a tarred and zipped file in devkitBase/include on macOS and Linux. Uncompress the headers in a directory and add this directory to your include path.
The version of Qt used by your version of Maya is listed in the Open Source Component page.