how do callbacks work

hi everyone, i have a handful of callbacks scheduled in the coming weeks and it’ll be my first time having an interview last longer than a screener. as i’ve never done this before, i wanted to ask wtf do y’all be talking about for 3+ hours during a callback. it feels so daunting to have to go through an interview for that long, so i just wanted general advice, tips, etc. i’m first generation and so all of this is new to me and i’m very nervous. thank you in advance!!!