MicroPython is written in C99 and the entire MicroPython core is available for general use under the very liberal MIT license. Most libraries and extension modules (some of which are from a third party) are also available under MIT or similar licenses.
You can freely use and adapt MicroPython for personal use, in education, and in commercial products.
MicroPython is developed in the open on GitHub and the source code is available at the GitHub page, and on the download page. Everyone is welcome to contribute to the project.