First post

This is my first year ever doing a garden. I've been trying to read up on this stuff but have a few basic questions regarding lettuce and onions. I've created a raised bed, filled it with soil, manure, peat moss, and miracle grow, and have gotten some of my plants from local farmers, some from local nurseries, and some from Lowes

Question 1: How does romaine lettuce grow? I bought an already started lettuce plant and put it in the bed. Will I just cut leaves off of the plant as they mature, or do I let the entire plant mature and harvest it? Will this plant give me lettuce throughout the season, or just 1 single head of lettuce?
Question 2: For onions, I bought onion bulbs and planted those. Will each bulb mature into only one onion? Or will one bulb give me multiple onions?
Thank you very kindly,
Bob Garden