No, 21570R15 the 15 means that tire fits on a 15 inch rim 21565R16 the 16 means 16 inch rim.
It provides a mount for the tire.
You cannot mount a 15 inch tire on a 15.5 inch rim.
No, the R15 means that it is a radial tire and the 15 is the rim size. Never try to mount a tire and a rim that are different sizes.
Of course you can.
On a machine specifically designed to mount tires on a wheel.
Mounting a tire on a rim by hand is extremely difficult. I would suggest taking it to a shop and having them do it.
no that would be a mount or a tire or a rim.
No, because inflation moves the edge of the tire outwards on the rim so it becomes tight.
the last two digits, the 16, indicate the rim size. as long as the rims size matches, your tire will fit (and lets be reasonable here, I'm not talking putting mudding tires on a car...)
Take the tire and the rim to a tire shop. It requires special equipment to properly replace the tire and balance the rim.
Any tire shop can mount and balance those tires for you