Can someone explain me the 50/50 thing?

I just got the game, since it's now available on Xbox. I finished collecting the 90 tapes to do the Yixuan run... And I couldn't get her.

How does it work? I have 10 days to farm everything again... Which I don't think will happen, since I barely know any method to farm, and I can install the game on my phone to use code or something like that...

I heard that if you fail the 50/50, the next character is guaranteed, is that true? If so, will my "guarantee" be saved for other banners? Or will it be reset from June 25th?

Thank you for your attention.