Yeah I've kind of butchered it, Il try and explain the method 1st step : Probability (at least two share the same birthday) = 1- p(nobody has the same birthday)

2nd step: let's just take two people, first person can have 365 days for their bday, second person can have 364 so the chance of them having the same is (365/365) *(364/365) then we can take this from one to get the chance that at least two font share the same birthday (quite low)

Now if we introduce more people then we continously have less days, so for three people you follow the same steps, and multiply your way down

After writing this I realised I'm bad at explaining, if you Google birthday problem probability much better explanations will come up