Since this repository contains mixed artifacts (code, documents, images, ...) each subfolder of the root directory contains its own LICENSE.md file which is applicable to the resources in the corresponding folder.
Because this is an open source project, source code artifacts and 3D models are licensed with the GPLv3 license while other artifacts (e.g. documents) are licensed with the CC-BY-SA 4.0 license.
But to be sure please check the respective folder for the used license.