Your supplier sends their digital invoice from their software to yours through the Peppol network. For this, they only need the unique identifier with which your business is registered on the Peppol network. In Belgium, this is typically the VAT number, in the Netherlands, it's the Chamber of Commerce (KvK), in France the Tiret number, .... You can find the identifiers for all available Peppol countries in the Peppol country overview.
If you have set up that UBL invoices can be processed automatically, the invoice will immediately appear in your expenses list. If not, it will first appear in the Fast Input where you can review it. In the supplier list, you can see which of your suppliers are connected to the Peppol network. Through the supplier file, you can even send an email requesting to receive your invoices from this supplier digitally in the future. More information about this can be found on this page.