It varies by culture and rite! The Greeks and Romans, I know, usually used a flat platform, where the animal would either be slaughtered on, or slaughtered just before and the offering (typically parts of the animal of marginal human utility) lifted up to the platform.

https://universes.art/en/art-destinations/jordan/petra/high-place-sacrifice/altar-1

I found this. My first strategy was to look up pictures of eid-al-adha (Islamic festival of sacrifice), and it seems like today people also choose to kill the animal on the ground separately from the worship ceremony. Which makes sense.