Whenever you submit work for a fixed-price project/milestone, or you finalize your hours at the end of a work week:
It looks like you are new here. I would definitely recommend that you read up on the help articles I will note below to have an idea on how the system makes it easy for you "send an invoice/bill" your client.
my question was different, is not how I get paid is how I manage to invoice to get the payment recorded in my balance sheet.
As said assuming I work through a limited company owned 100% by me (pretty common in UK for example) should I invoice UPwork? how that works?