You can export a BibTeX file from both the desktop and online versions of Zotero. However, the desktop app offers more control over your .bib file (e.g. excluding notes).
Save the generated file in the same folder as your LaTeX document.
If you are using Overleaf, click on the Upload button, then select the .bib file to add. You can rename it at any point, but ensure that the file is saved with extension .bib.
You can add references from your existing BibTeX file into your Zotero account. Similar to exporting, the desktop version offers greater control over import options.
Don't forget to review the newly imported collection to ensure all references have been added correctly.