All the supporting documents are scanned and uploaded once you have paid for the visa. At that point you will be directed to the Sopra Steria site where you make your appointment to enrol your biometrics. These are no longer done at the Post Office but at one of the Sopra Steria locations. Some core centres are free but others charge a fee depending on the services you require. You can have your documents scanned and uploaded at some centres but you pay extra for that. In any case, it is better to scan and upload before the appointment. When you have paid for the 'visa' you will be given options for further services, such as document checking.
At the Sopra Steria appointment, your original documents including passports (scan every page) and BRP will be returned to you, but you cannot travel while the application is being processed. You do not post any documents in the new system. If you want a speedy approval, you pay £800 to hear within 24 hours.
You do need to submit passport size photos any more.
I hope this clarifies the new system, which seems to be working well.