Where is the scheduler in distributed learning?

Hi, I have a question related to the distributed learning scenario.

While I am doing distributed learning and specified only the number of workers.

Based on the description here:

Each worker node will also specify a server process.

However, what is the role of the node launch the distributed learning job?

It is a or a server or schdueler?

Also, if we have more than one server process, does it mean we are also using model parallesim ?

Best Regards,