In a family business Odoo is usually the central system; in a larger group it tends to sit at the edge. We connect it to banks, to the Peppol network for invoices to Government buyers and trading partners, to online sales, to couriers and to payment providers, and within a group to the SAP core for consolidation and intercompany flows.
The external API of Odoo covers most connections on its own. When volumes grow, or several systems depend on the same records, an integration layer goes in between, so upgrading one system leaves the others working. Those interface contracts are designed by our integrations and API team.