We have seen famous Social Networks like Facebook being made using Open Source Programming languages like PHP and then transitioning into more proprietary technologies. It is fairly common that new technologies get built on top of existing architecture then as development progress other proprietary tools get built and facilitate how business is done.
More sophisticated System Architecture like Microservices and Distributed Databases becomes contenders for scalability. The culture of Open Source Software is very generous in most cases, it allows Developers to interface with core libraries on an Open Source License Agreement and requires not to change any pre-built source code. However, if the change is made to the Source code of the Open Source Library then it is required to make that modified part of the code public.