The xtUML Editor stores model data in text files with file name/extension *.xtuml.
The contents of the model are stored in multiple files divided by package. (This is done for scalability to make it easier for large teams to concurrently work on the same model and minimize conflicts.)
Inside the files you will see SQL INSERT statements. During load, the model data is inserted and related in-memory.
The schema for this in-memory “data base” (actually linked java classes in the plugins) is the xtUML meta-model (OOA of OOA in days of yore). The meta-model and schema are open source and published in various places. In the xtUML Editor you can find the meta-model as an example project. (Hmmm, the editor is delivered with a model of itself.)
Using Export from the project menu you can export the entire model to a single file.
A programmer can also access the model data through a java API (which is generated from the meta-model)