Reference:
When a device boots up with the startup configuration, the nginx process will be running. NGINX is an internal webserver that acts as a proxy webserver. It provides Transport Layer Security (TLS)-based HTTPS. RESTCONF request sent via HTTPS is first received by the NGINX proxy web server, and the request is transferred to the confd web server for further syntax/semantics check.
ios/prog/configuration/168/b_168_programmability_cg/RESTCONF.html
The https-based protocol-RESTCONF (RFC 8040), which is a stateless protocol, uses secure HTTP methods to provide CREATE, READ, UPDATE and DELETE (CRUD) operations on a conceptual datastore containing YANG-defined data -> RESTCONF only uses HTTPs.