The Nexus takes JSON requests and translates them into API calls for the respective real bank accounts (EBICS, FinTS, …). It also stores the bank transaction history to enable a linearlized view on the transaction history with unique transaction identifier, which some of the underlying banking APIs don’t provide directly.
libeufin-nexus-httpd
is the binary (or wrapper around the Java invocation)
that runs the HTTP service.
The Sandbox and Nexus are only HTTP services. The CLI tools are used to actually access them.