![OSI Symbol](/sites/default/files/styles/content_image_fluid_sm/public/2022-08/OSI%20symbol.png?itok=B40E4X1g)
The quick answer: the underlying source code of the system is publicly available. For most open source projects, that also means that the development is done in public, everyone can contribute to the code.
That does not mean that anyone can just change things. There are still maintainers who make sure that everything that goes into the project has the same high standards. Often, this also means that this software is of higher quality and can be obtained without licence fees.