While consultation for customers in Japan, there were cases where they desired to implement custom external repository connectors, but we found out there was no documentation for these. It'd be great if there is documentation, even high-level from a developer standpoint.
The reference implementations could be found in liferay-portal such as S3Store.java, and I assume developers can guess the location from the current documentation, for example from this https://dev.liferay.com/es/discover/deployment/-/knowledge_base/7-1/using-amazon-simple-storage-service , but they aren't enough for custom implementations.
Especially in a case to connect multiple repositories for Documents and Media, ExtRepository needs to be implemented, but no reference architectures as BaseStore available in the public due to reference implementations are private.
I referred to DocumentumRepository and SharepointWSRepository. Some documentation, ideally reference implementations such as blade sample would be helpful