How would Blackrock's Bitcoin Spot ETF affect the market?

I know the overall sentiment is that the ETF being approved is extremely bullish for the market, but I don't understand enough about how ETFs work for it to be very clear to me. From what I understand, the ETF is basically a way for investors to gain exposure to BTC without actually owning any, so they aren't exposed to the same risks as a regular holder (owning a share of the ETF opens its own can of worms, but that's a topic for another day). As a result, Blackrock would have to own the Bitcoin itself for it to reflect within the spot ETF.

However, I have several concerns about it. If all it takes is for Blackrock itself to own the Bitcoin, then couldn't they have simply slowly accumulated over time? And now that they own a satisfactory amount of Bitcoin, they can try to get a spot ETF approved that's based off of their Bitcoin holdings. If that's true, then I think the market would already be fairly priced in, and the ETF being approved wouldn't affect the market too much in and of itself. Obviously, there's going to be some price movement to the upside just off of the news of it being approved, but I'm unsure as to how bullish it'd be if Blackrock already accumulated all the BTC it needs.

I don't know if Blackrock's actually allowed to accumulate the Bitcoin they base the ETF off of before it gets approved, so I'm just checking to see if my assumption is correct. Feel free to correct and educate me on how ETFs work.