could someone explain how the bullets works in Westworld

IIRC, the guns and bullets are real, but they can't actually shoot a Guest due to programming.

We saw the bullets used against the Man in Black a few times in Season 1, and he didn't get hurt at all. So, they appear to impact, but they don't penetrate the skin. With the safety system offline, park-wide, the bullets act like regular bullets now.

I thought I heard someone say that the hosts got a hold of the real ammo. Weren’t they armed with blanks or pellets before? IDK, I could be wrong but that is what I thought was going on.

in the future, everything is magic.

just rewatched again with subs and found my answer, in the stable scene Hale ask Bernard about the guns and it seems that Ford had made some changes to the system so it identify all the humans as hosts. But how the system works with the guns to ensure the safety of the guests is still unknown though.

I've tried to get my head around this back in season one. My best guess is each bullet has an internal projectile that is fired on contact with a host but is inert if it hits a guest. Each guest has some sort of transmitter that deactivates the secondary projectile if it detects a gun aiming at a guest. This allows the on the fly re-designation that all guests are hosts that they have explained.

It's a plot hole. At first they wanted to make the park magical so the bullets bounced off. Then they changed their minds and decided it should be real so the bullets now kill.