Some B-flats made completely of metal can be found. They are built in one piece exept for the mouthpiece joint, and you will notice that the metal clarinet is much thinner than the wooden brother. Metal was mostly used in the early century in the US, but is doubtfully constructed any more. Plasics are used in cheaper school instruments but are also used by marching bands when they perform outdoors. Key mechanics are plated with Nickel or Silver on the more expensive instruments.
Silver gives the best feeling and precision.
The conductor reads the music from the score with all parts included. Very often the score is written with all parts in the same key signature. They read the notes as from the score. The clarinet however is pitched in B-flat. If you would play from the score, the note C would actually sound like a B-flat played on the piano.