The jQuery.sap.require function is contained in the jquery.sap.global module, which requires jQuery.
Both are included in the SAPUI5 bootstrap files sap-ui-core.js and sap-ui-core-lean.js. The first one also embeds the SAPUI5 core functionality and needs no further modules. The second one only contains the two bootstrap modules and a require statement for the core. For loading modules separately, the sap-ui-core-lean.js bootstrap file is better suited.
If you create a new bootstrap file with the configuration UI and decide to include the jquery.sap.global or jquery modules, they are always the first modules in the created file and are always embedded. This ensures the availability of jQuery.sap.require.