The answer will make you die a little. I put on a pot of tea, and I do it manually…. It usually takes a few hours for <100 citations.
But, the answer makes more sense when you learn that my employer is a Google Workspace. Google Docs is rubbish at footnotes/references, and doesn’t handle them in a way that seems very compatible with other apps, so every time I export to a different format it all has to be manually checked anyway (the trick is to export to Microsoft Word first, which formats it properly, then convert that doc to whatever is needed. I often forget this until I’ve made a mess.). (Google Docs has other benefits, e.g. having multiple authors working on a doc at once is a breeze, but it definitely does not shine with referencing.)