Typical external systems that are usually integrated with Magento are:
Payment: Stripe, Zoho, Paya, Opayo, Worldpay, Moneris, etc.
Logistics: DHL, Viettel Post, Grab Express, Ahamove, etc.
ERP and Accounting: OdooERP, SAP S/4 HANA, SAP B1, MS Dynamic NAV, etc.
POS: NAV LS Retail, Retail Pro, etc.
CRM: Salesforce, OdooCRM, ZohoCRM, Bitrix24, SageCRM, etc.
Server & Hosting: AWS, Google Cloud Platform, etc.
GMP/Customer Engagement Platform: Insider, Salesforce Marketing Cloud, Pardot, etc