Calculators for decimals, like all calculators, perform mathematical functions by converting our base 10 numbers into base two numbers and then performing the requested function. Conceptually, decimals seem different than working with whole numbers. However, calculators handle them the same. The only added step is calculating the decimal place.
To understand calculators, you must first understand binary. Binary is a number system with only two numbers. 0 and 1. Counting in Binary would look something like this. 0, 1, 10, 11, 100, 101, 110, 111. Calculators have hundreds of little switches that are set to either off (0) or on (1). In order to handle numbers, the calculator converts a number into binary so it can work with it via a variety of processes that involve switching the array of switches on and off.