Your founder must build their house, build their wealth, and ultimately bring in the second generation to continue when they die. The Legacy Challenge tasks you with creating a single founder Sim moving them in to a giant lot, and starting out in abject poverty. By abiding by the restrictions of the challenge and reaching for the goals of the challenge, you will find yourself getting a very different experience from The Sims 4 that you might have playing casually.
The Legacy Challenge is a set of self-imposed rules meant to change the way you play the game of The Sims 4. The Sims 4 Legacy Challenge Rules (updated November 9, 2022)