Side-by-side extensions are software application running externally to the core application (SAP S/4HANA Cloud) and interacting with it using SAP standard APIs.
Side-by-side extensions provide a freedom of expression and do not suffer the restrictions of in-app extensions.
They can potentially run on any run-time application platform, nevertheless SAP provides and recommends it's dedicated platform: the SAP Business Technology Platform.

Side-by-side extensions can be coded using potentially any programming language, nevertheless some languages are better supported by SAP, in particular ABAP, Java and JavaScript.