Ammo varies by airline. Some permit none. Others permit a limited amount. Check your airline's website.
In some airports, the airline ticket agent will check you in and then put a check tag on you luggage and instruct you to shlep your bag to a TSA check point to submit it for inspection. In other airports, the airline ticket agent will take your bag from you and forward it to the TSA for inspection without further effort on your part. If you are flying first class, the ticket agent usually takes your bag or at least summons a skycap to carry it to the inspection point for you.
No major airport has been built in the US in the TSA-Era. So, no airport in the US is designed architecturally to accommodate the TSA inspection of checked baggage. Each airport has had to figure out how to cobble up improvised procedures to shoehorn the TSA into its originally-designed flow. What they came up with necessarily varies from airport to airport. Here in Portland, at PDX, for example, the airport has an elaborate system of conveyers that were part of the original design and which used to efficiently move all of the baggage from the ticket counters to the sorting area. That system now sits idle. Passengers shlep the bags to the TSA inspection areas. After inspection, porters with carts manually move haul the bags to the aircraft.