I'm pretty sure vercel only counts bandwidth from the request payload from user and the response payload.. so in this case you uploading to S3 won't be counted, but you may still get charged for the ingress... (This is at least how it was last year, I havent checked again)

Uhh look into uploadthing as it stores in S3, but it only uses your server to validate aith of user to upload (not actually getting the file)

you should use presigned url here

you obviously can't send TBs of data to api routes on vercel, your bill will bankrupt apple