In a Spot, Bitcoin ETF invests directly into Bitcoins, meaning they must have bought bitcoins at one point before they filed and got approved by SEC. Do you think SEC would go that low into approving an ETF the Government still hates, minus following the actual rules to the book? I don't think so.
Maybe you are confusing Spot ETFs with derivative based ETFs or futures contracts.
The first spot Bitcoin ETF was applied for a long time ago
Will they buy again a very large sum until they run out?
Will they then buy everyday?
Price spikes!
There is always someone willing to sell at a higher price. That's how the market works. Exchanges don't run out of assets to sell, it's a game between the buyers and sellers. It's all about demand and supply.