I know that pi contains every finite number sequence, but i am not sure why. Since you can take any irrational number and change every $9$ to an $8$, then clearly not all irrational numbers are the same way, since the resulting number won't have any sequence with $9$ in it. Then what nunbers are this way, and how can you show that its true for pi (or any other number)?
The numbers with that property are called normal numbers in base $10$. And nobody knows whether or not $\pi$ is normal in base $10$ (or in any other base). A number which is normal in base $10$ is$$0.12345678910111213\ldots$$