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It is not they who choose it, it is he. They are only too happy to let him do it, and even for that need an introduction. Mme B., a personage of the Bean Mounde and elegant to boot, went to him last month to order a dress. "Madame", he said,"Bye whom are you presented?" "I don't understand." "I'm afraid you must be presented in order to be dressed by me"

I would like to get clear meaning of these sentence. "it is not they who choose it, it is he." and "even for that need an introcution" can you write them in another form. and what is "Mme B", "Bean mounde" and "elegant to boot".

with all these thing the paragraph really difficult for me to understand.
Comments  
It is not they who choose it, it is he. This means that he himself made the choice. Not someone else. No one forced him into the choice. He made the choice.

even for that need an introduction. This implies that 'they' are timid, or cowardly, or lazy. So let 'him' do it. But I am not sure without more text.

Mmb B. [French for 'Mrs', I think...Mrs B. Some anonymous person]...a personage of Bean Mounde [an important person of the Bean Mounde family OR an important person of the Bean Mounde area]...and elegant to boot. [refined, learned, intelligent, upper class...also. 'To boot' means also.]

So some really good dressmaker is picking (or making) fashions at the time of this story. So, I will rewrite the passage:

It is not the people who wear the clothes that make the fashion, it is the dressmaker. And the people are happy to let him determine what will be in fashion. But he is so much in demand as a tailor/dressmaker, that people need an introduction to become a customer. Mrs B, a person of importance in Wales (wherever) and already dressed finely (elegant) went to him last month to order a dress. "Madame," he condescendedly said (The dressmaker is esteemed higher than Mrs. B and the dressmaker KNOWS it.), "hit the road. Come back with an important person, that I know, who is willing to vouch for you."
thanks for full explanation.