(a) does mailman make a copy of the message to each of the recipients (as majordomo does) or does it work in the form of a (listserv) job such as

i.e. sending the message with distribution list to a server and "exploding" the contents due to some routing tables arrangement, saving much bandwidth and cpu time, etc. ...

yes, although I can't believe Majordomo doesn't do that as well. Mailman will send mail to N users using M SMTP transactions (or invocations of sendmail if you have it so configured), where M = N / (some constant).