Pi is a constant (never changes) because it is a ratio. It is the ratio of the circle's circumference and its diameter. The larger the circumference, the larger its diameter. So Pi will always be the same. A ratio is basically a division of two numbers. To make it simple, think of a circle as having a circumference of 10 and a diameter of 5 (Remember, this is not true, but I am making it very simple). The ratio would be 10/5 which is 2. A circle of 20 circumference would then have a diameter of 10. 20/10 is still 2. So, now change the value to real circle and divide the circumference by the radius (like I did), and the answer would be 3.14, which is Pi. It is always going to be 3.14. By the way, Pi also equals the Area ratio to the radius squared.

They use the fraction 22/7 but pi is rounded

Pie is a number lasting forever. And 3.14 is the first 3 digits,which they decided to use.