In Option-Chain different strikes have different IV. But Why? when VIX is the same?

How do they calculate IV? If we use Black-Scholes Model to calculate options prices. The Black-Scholes model requires five input variables: the strike price of an option, the current stock price, the time to expiration, the risk-free rate, and the volatility.

Out of five variables, four are easy to collect. but How do I know what is the IV?

for knowing the IV we need the Option price. for calculating options price we need IV. In an equation How is it possible to get two variables at a time?

Iām totally confused .