Question for the devs here, All the voice ai companies out there like retell, vapi etc, they have this workflow builder to customize the conversation workflow. I am not sure how internally they make the llm to follow that, do they just transform the whole workflow into one big giant prompt or what, i am trying to do something similar for my product (not voice related), i am not sure i am following the best practices here. LMK if you know anything about this.

The specific internals are, by the most part, assumptions. But, almost certainly, it is not a big prompt. Most likely the prompt and tool set is changed (likely partially) when it moves to each node. If you want a workflow builder reference for best practices, n8n or dify are good places to start