how would a man become a brother, and is it the same as being a monk?
One would join a religious order that has brothers.
Technically monks follow the rule of St Benedict and are cloistered. Brothers are usually of a mendicant order and are not cloistered but active.
There are other groups that are not mendicants who have brothers.
There are a couple of groups that only consist of brothers.
For the Carmelties, one would speak to a vocations director and express that they are interested in only being a brother. Of the six of us currently in the Novitiate one of the men is going to be a brother the rest of us are on the track for the priesthood.