Master cylinder is low of fluid or is defective would be the first thing to look at. Can also be a leak anywhere in the system.
It wasnt assembled right
A brake fluid leak in the system would do that. Check the brake fluid reservoir. If it is empty you are in need of brake work asap!
The first suspect when a brake pedal goes all the way to the floor is an empty - or almost empty - master cylinder. It could also be a bad brake booster, a ruptured line, etc. The car should be TOWED to a repair facility, and the cause found and corrected before the car is driven again.
The gas pedal, the brake pedal, and the steering wheel all do.
Defective master cylinder. Replace it ASAP
The brake light switch is out of alligment. Check on the brake pedal arm and readjust the switch.
When you press and hold the brake pedal it will slowly move to the floor. It can also have a catastrophic failure and go straight to the floor with no stopping ability at all. If you hold the brake pedal down and it slowly moves to the floor any at all, replace the master cylinder.
It is above the brake pedal not under it. Follow the brake pedal bracket all the way up and you will see the brake pedal switch.
The gas pedal, the brake pedal, and the steering wheel all do.
You have a leak. It could be from the master cylinder, lines, calipers, hoses, or wheel cylinders. Add fluid, pump the pedal, and look for the leak.
Master cylinder failure.
Check your brake fluid level in the master cylinder... you may have a leak some where