Margin Investing Robinhood App

Not seeking financial advice, just theoretical discussion and the numbers used are just for simple math.

Can someone explain to me very simply how margin investment works?

If i enable margin investing, do i automatically have to invest it?

For example, if i’m eligible to take out $1million at 4.5% in margin funds for investing, can’t i just invest $5,000 a month for the next 16.67 years into the sp500 (historically super low risk, specifically for a longer term). What am i missing here?