New to ETW a few questions

[Valion]

Hello there i have a noobish question

Do newer rates of ships replace older ones? Can you still build old ships late game? like the 5th rates? i ask coz i noticed sakers get replaced by 12-lber Foot Artillery in the recruitment list.

If i allow the 13 colonies to become the United States will they eventually build the USS Constitution?(yes i have the DLC)

I can't for the life of me find a use for Brigs., can more experienced players tell me what you use them for?

Thanks in advance!