Bitcoin private keys uses SHA-256, which is the number 2 to the 256 power.
Where did you hear that? I hope your youtube video is not misinformed as you are, because sha256 has nothing to do with private keys. And if want to know how hard it is to "guess" or generate a key twice, considering your wallet is using standard implementation, you can write down 78 digits in decimal and see how hard it really could be.
Your number which you can pick, must not have first three digits greater than 115. Because this is the limit, any number greater than this would be invalid:
115792089237316195423570985008687907852837564279074904382605163141518161494336
This is meant to be a simplified explanation in lay terms, not a very technical one. Most of us don't have time to read a textbook or take classes on the math and science behind hashing algorithms and mathematical theory.
But to say that sha256 has nothing to do with private keys shows that you have zero clue about what you are talking about and I will not bother to explain it to you.
Go sign up for some classes and you might learn something.