Detailed understanding of
EJB? Not really.
However you do need to know/understand the 5Ws of the technologies in the
Java EE platform, ranging from
servlets and
JSP to EJB, JMS, JTS, JPA to
SOAP and REST.
The purpose of the Architect MC exam (part 1) is to
test you the design principles, when to use what and why, tech pros and cons, design
patterns, security etc.
Base on that understanding and experience, the assignment/essay (parts 2/3) tests you the approach of applying the part 1 material, defending your design decisions in the essay.
All in all there will be lots of reading. Knowing how to develop web apps or EJB should help you understand the core of that technology.
During my preparation for JEA5, I ended up doing all the EE5 developer exams. Among those, I found EJB material the most useful, allowing me to appreciate the core of the JEE platform.