How come my hardwired smoke alarms are chirping?

Hi, it means that the backup battery is running low. All fire alarms have batteries whether hard-wired or not in case of power outage (which is probable during a fire).

If replacing the back-up battery does not solve the problem, then it means the alarm is malfunctioning and should be replaced.
Since you have multiple alarms chirping, then it's probably that they were fitted at the same time and the batteries are at about the same charge. Sounds like it's time to replace the batteries.

No idea sorry, it should be obvious though. Open up one of the units and have a look inside. It's probably a round watch type battery. It should be fairly obvious otherwise some people would have no hope of replacing their batteries!

(this is a guess!)

What kind of detector do you have? Most that are ceiling mounted twist off counter clockwise about a quarter turn and then drop down. If powered by AC, then there will be a wiring harness attached to the back or top of the detector. Here you will find a small door or cover that needs to be opened to reveal the 9 volt battery.