There are no proprietary, DEVONthink-only, file formats used in DEVONthink, meaning all documents created in the database are one of those formats you see when you look at Data > New. Export your rich text document - or drag it out of the database and into Finder - and it will be RTF, including your images, text, and tags.
A tip: in 3-pane view, or list view, add the “Kind” column to the panel that lists documents and you’ll see the file format that each document in the database has.
The tags for exported documents are in the OpenMeta format and are available to other applications that support OpenMeta, including Spotlight searches.