The original version of this document is located at at https://docs.qfield.org/get-started/license
QField is released under the GNU Public License (GPL) Version 2 or above. Developing QField under this license means that you can inspect and modify the source code and guarantees that you will always have access to a QGIS based field data collection app that is free of cost and can be freely modified.
You can find the source code on github