eli5, is it true that irrational numbers like pi contains every single number combination in it?
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Not necessarily. A number could be irrational, but not contain every digit, never mind every combination.
An irrational number is just a number that you can’t make as a fraction of whole numbers. It’s basically one that has infinitely many digits, but doesn’t settle into a repeating loop. For example, 0.33333… is a rational number because it’s just ⅓. Or, you could have 0.142897142897142897… where those 6 digits repeat. That’s just ⅐.
If an irrational number has roughly equal amounts of all digits, and there’s never a point where it just stops using a particular digit, then we call it a *normal* number. In this case, going on forever and not settling into a repeating patter and using all the digits, any finite sequence will eventually appear.
Okay, so how could you have an irrational number that *isn’t* normal? Well, let’s take the number 0.1101001000100001000001… where each group of 0s just has one more than the last. It’s irrational because it doesn’t fall into a loop (although there is a pattern there). But it doesn’t contain any sequences involving any of the digits 2-9.
We *think* that pi is a normal number. But we don’t know that for sure. It could be that after some point, it just stops using the digit 8. It’s an open question in maths, but most people agree that it’s probably normal.
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