In which type of extensibility is the SAP Business Technology Platform used to host a...

Comprehensive and Detailed Explanation From SAP S/4HANA Cloud Public Edition, Management Accounting documents:

Side-by-side extensibility uses the 62SAP Business Technology Platform (BTP) to host custom extensions that run outside the core S/4HANA system636363. These extensions interact with the core via APIs or events64. This approach supports a "clean core" strategy, ensuring that extensions are decoupled from the main application and are stable during release upgrades .