I second what @Benartha_Verly says. If you are looking for the same kind of integration, this is going to be difficult. The API is called WOPI and is also used by Microsoft’s online office suite. Seafile implements a subset of WOPI to interface with LibreOffice and Microsoft, and a different API for OnlyOffice.
However, under some circumstances it can be easier. I have written an online editor that loads the files from Seafile and saves them back, but is otherwise not tightly coupled. It has its own directory browser and you cannot link between them. If you want to open a file in that editor, you will first have to open the directory in its own directory browser, not in Seafile’s web interface. The second limitation is that it does not allow simultaneous collaborative editing. The interface between Seafile and the editor is via Webdav. But it could also be the Seafile API. Both are easy to implement.
If these limitations are not a show stopper for you, you could consider this. The editor remains incredibly useful for my group. You can take a look at it, it’s on GitHub: https://github.com/MJochim/webdav-editor
You can find some documentation on the APIs here: https://download.seafile.com/published/web-api/v2.1