Xournal++ is an open-source and cross-platform note-taking software that is fast, flexible, and functional.
It includes many features and it can be extended via plugins. Xournal++ saves it's notebooks as an xopp file.
Currently (2022-09-01) there is no way in the app itself two merge two or more xopp files.
This web app was made to make merging xopp files possible and simple.