How deep is the diligence process in ETF/Mutual Fund Investing?

Prospect here asking how time is spent in AM (Mutual Funds / ETFs) rebalancing / monitoring positions as opposed to doing investment diligence / new opportunity analyzation?

I'm Currently on the sell side and want to break into the AM space but I want to understand the investment process much better and what goes on behind the scenes.

I'm hoping it's as diligent and well thought out as Private Equity investing is but would like some color from people in the industry around the investment thesis / diligence portion and truly how deep Mutual Fund / ETF Managers go.

Would you say that the mega AMs like Blackrock, Fidelity, T Rowe Proce don't do as much diligence as smaller ETFs managers do like Ark / O'Shares others.. thanks in advance!