How would you invest $1M for passive income/cash flow?

Vanguard Index funds if you want something truly passive, real estate if you don’t mind some effort

Specific ideas are welcome especially if you been doing this already.

I would reject the term "passive income" as not helpful and think about how to create a portfolio with good risk-adjusted returns, regardless of whether they come from interest, dividends, or capital gains. The withdrawal rate of the portfolio, which produces the "cash flow", is a separate question.