When you buy stocks are you investing in the company directly and thus they benefit?

For example lets say i didnt agree with Nvidia ethics, if i bought the stock am i buying it from Nvidia or from another investor?

When a company is private but they go public and have an initial offering, is that the only time where i would be investing in Nvidia directly?

Stocks only benefit the major shareholders right? Its why for example major investors of Tesla are wanting Elon to step away?