Please I want a correct answer to these questions. Why is there no expert who answers these questions in a correct way other than those on the site?

1- Why does a manager need to valuate for a project or for a firm? 2- How do you choose a project for the investment, if you are a manager in a company ? 3- What important inputs does Discounted cash flow valuation require and why they are very important? 4- How to estimate a firms weighted average cost of capital (WACC)?