I'm a gateway for singles. Sometimes I have bars, sometimes I look like a type of door. You will usually encounter me at the start and the end. I thrive in subway or metro stations. Can you guess what am I?

Three snowy spheres all stacked with care, a carrot nose and scarf I wear. With coal for a smile, I proudly stand, a winter guest shaped by your hand.

In waves, in lattes, and beds I’m found, a crowd of bubbles packed around.

First three have sharp teeth To cut logs that are beneath Last four mean a factory in brief A place to shape timber to lumber, its function chief

I sound like complete harmony. I play board games. Technically, I'm only a portion but I have an entire pie in me.