Minimally, it should be obvious how to import transcripts from NVivo Transcription and match with an audio file in a project.
I almost couldn't find the "Import rows" button (which only shows in the ribbon on NVivo 12 Plus when the audio editor is already open and not on any context menus) to deal with the transcripts I requested through the web portal. I don't know how it works yet for transcripts requested through NVivo's internal login, but there are plenty of situations where you would want to add a transcript to a different project, etc.
Ideally, there should be a seamless (no download/upload) connection since NVivo can already connect through a direct login. You should only need download/upload if the transcripts are on someone else's account.