BibTeX is a file format used for lists of references for LaTeX documents. Many citation management tools support the ability to export and import lists of references in .bib format. Some reference management tools can generate BibTeX files of your library or folders for use in your LaTeX documents. BibTeX styles available on Overleaf can be found here. Wikibooks also offers a page on bibliography management. This is video #9 in a series of twenty one by Dr Vincent Knight of Cardiff University. Alternatively, you can read the article found here that address bibliography management with BibTeX from Overleaf itself.
Find resources for linking Overleaf to a reference manager such as Zotero or CiteULike here.
Specifically, you can import your bibliography from Zotero or CiteULike. You can get a CiteULike ID by following the instructions from Overleaf found here.
Mendeley is another reference manager tool, similar to Zotero. You can link your Mendeley to your Overleaf account following the instructions here. If you have a group on Mendeley, you can also link that to your Overleaf, as seen here. Please note that some reference management tools in Overleaf are only accessible through a Pro or paid account, which is why it is important to use your CMU account when signing up for Overleaf.