Mendeley has two different desktop apps, an older one (awful UI) still supports BibTeX keys and you could use something like Pandoc. But their new app (even more awful UI) removed even this and as I mentioned in another thread, when I asked their support about workflows other than being locked into Word/Libreoffice plugins, I got corporate BS replies.
Apart from Zotero (which can integrate into a Scrivener workfow), JabRef is another alternative. It is a great reference manager (very comprehensive ref editor, much better than Zotero etc.), free and cross-platform. While originally for LaTeX workflows, it does support formatting LibreOffice docs and so you should be able to plug it into a Scrivener workflow too… https://www.jabref.org
For completeness, a Scrivener + Pandoc workflow gets you a complete bibliography + citations solution without any need to scan documents etc. If I was on Windows I would use JabRef to manage my references and Pandoc to convert Scrivener output…