If I understand correctly the default JNDI service runs on my local AS, right? So if I create an EJB and in jboss.xml (running JBoss) I name it "sth" than it is registered in my AS. Correct?
In big projects EJBs might be distributed through many servers - on one server EJBs doing sth and on another sth else. When calling JNDI loopup() I search only one server, right? So it means that I need to know where the EJB is registered... Is it true?
When you cluster your app you will usually configure the cluster so that you have one shared JNDI. In JBoss you do this using HA-JNDI (High Availability - JNDI) or equivalent. This is a centralized service with fail-over. In principle you could imagine having a replicated service for better throughput, but to my knowledge that is not available in JBoss.
In short, you will have only one namespace, so you don't need to know where it is registered.