If you have a list of formatted citations, in a published article, or you own manuscript, you can use the AnyStyle.io parser to create citation files that you can add to Zotero.
Note: It is very likely that the data you get from the parser will need to be cleaned up if you intend to use the data to create new citations.
If you are working with a small number of citations that have doi numbers or ISBNs, it may be easier to use the Add by Identifier tool to add your citations to Zotero
Depending on your device settings, you may need to go to to import your citations into Zotero instead of just opening the .bib file from your Downloads. Follow the steps described in the page on importing from other citation managers.
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Questions? Contact your subject librarian.