The builder takes four inputs: how much capital, what return you are aiming at, the minimum probability you will accept for reaching it, and the maximum number of holdings. Those four decide everything that follows.
Raising the target return while keeping the minimum probability fixed narrows the set of portfolios that qualify, and the ones that survive tend to be more concentrated and more volatile. Watching that trade-off move in real time is the fastest way to understand that return and certainty are bought with each other.
The output is not just a list. You get the weight of each position, the expected return and probability for the basket as a whole, and the contribution of each holding to total risk. That last column is the one to read first: it shows when a 6% position is quietly driving 20% of the risk.
Treat the result as a first draft. Override a weight, drop a name, add one of your own, and watch what happens to the joint probability. A portfolio you have argued with is one you can defend in front of a class; one you accepted unchanged is not yours.
Two constraints are worth setting even when the tool does not force them: a cap per sector and a minimum number of holdings. Without them, optimizers tend to pile into whatever looked best recently, which is exactly the behaviour a challenge should teach students to resist.
Educational material. Not investment advice.