Can someone explain why the probability of getting a royal flush is only based on the choosing the suit?

Question

So $|S| = 52C5, |E| = 4C1$ therefore $P = \frac{1}{649740}$. If you only select the suit, how do you guarantee that the cards OF the suit will be 10, J, Q, K and Ace? It seems so random that only $4C1$ is required and I don't understand. I know that the cards would be "fixed" since you MUST have the 10 at least. But even so, I am questioning it.

Answer 1

$$\text{Probability} = \frac{\text{N(umerator)}}{\text{D(enominator)}}$$

where $N =$ # of successful 5 card combinations

and $D =$ total # of 5 card combinations.

$$D = \binom{52}{5}$$

and

$$N = 4 ~~\text{(i.e. one combination of AKQJ10 for each suit)}.$$