I second Trading and Exchanges, a phenomenal overview for folks who want to learn how brokerages and market making work.

I wonder what it feels like to click send on a 13B + transaction.

I ride a bike in NYC. If you accept being sweaty in the summer and risk getting hit in certain areas, then you'll have a blast riding. Otherwise it's a burden. In my opinion, biking is one of the fastest forms of transportation when traveling <40 blocks. Also, Citi Bikes are lunky. I hate riding them.