When a website takes a Mobile Money payment, the payment gateway's API is what it calls. When a CRM updates a spreadsheet, an API carries the record. Almost every integration is two APIs and the logic between them.
A system without an API can only be integrated by fragile means such as scraping or manual export. Choosing tools with good APIs is what makes a business's systems connectable later.
Every integration we build starts by reading the API documentation of both systems, because what an API cannot do sets the limit of what the integration can.