The Arduino (and many other boards) have a very useful time-and-cost saving feature ideal for when you are working with buttons and switches - namely internal pull-up resistors on the GPIO pins which can be enabled / disabled at will in software. This means you don't have to clutter your board up with pull-up resistors of your own for all the buttons and things, and also means they can be turned off and on to give your design much more flexibility.
But nowhere can you find out actually what the resistance of these resistors are. Why not? Simply because nobody knows.