Off the plan advice needed pls

I am seeking advice on my options to sell soon to be sub-divided property. I have little understanding of off the plan.
Q1: Can someone explain how it works,pls??
Q2: How early in the project does the vendor get paid? How much of purchase cost? All? Only deposit? I am wondering how to access funds asap on 1st sale to help fund the rest of construction.
Q3: Does the builder need to be involved somehow?? I am told the “builder will know”! But I dont want to leave it up to him![biggrin]
any advice would be appreciated.