A lot of countries are using Peppol but some countries have their own system. If you're using Billit, that's not a problem: based on your customer's identifier the platform will choose the correct format and network for sending. Please note: some countries require you to register first.
You can find an overview of all countries you can exchange invoices via Billit in this country list.
You can read per country how you can send an e-invoice.