not every shiny app is the same.
Some are hosted on your own server, where you are the master.
Others are hosted by other organisations where they rule the roost, and their own restrictions may apply. I.e. the temp disk space where an uploaded file would be landed.
According to this article the default limit for the upload itself is 5mb, and you can adjust it:
[Shiny - Help users upload files to your app (rstudio.com)](Shiny - Help users upload files to your app
I would be pessimistic about using a 1Gb file upload in a workflow unless the users where in the same local network as the app host, I think it might be quite painful to have users over the wider internet try to upload 1Gb.