I am interested in an approach how to use the external link for collabora without Nextcloud. As I can see in the loolwsd.xml, there are three types of storage, namely file, wopi, webdav. Instead of them, for example, I want to use http://my.external.storage/test.doc to the link http://localhost:9980/loleaflet/dist/loleaflet.html.
Is it possible to use a direct link to my file, or just an extra layer approach like Nextcloud might be a suitable solution?
Direct file access is only for testing purposes, and WebDAV access was an idea a long time ago that never got implemented, so yes, a WOPI host like Nextcloud is the only advised option for any real-world use.