The invoices must be automatically sent to the tax office platform from your manager program where create your invoices.
This should be done immediately when you create an invoice or at regular intervals (every 15 days for example). I have seen some programs that have a button in their program and when click it, the invoices automatically sent.
Tax office are giving an API for programmers to configure and connect their programs to the tax office platform.
Here the Links for the API
https://mydata-dev.portal.azure-api.net/
Unfortunately it writes them in greek.
I am not a programmer and I cannot understand more.
Everyone must connect their program up to 1 july.