>Some other index fund sees that the price is attractive
Index funds do not have opinions on the attractiveness of prices. What happens in this hypothetical when one index fund has a cash inflow is that it bids on all the assets it is "short" of, which increases the price until the other index funds have "too much" of the now-higher-priced asset and decide to sell.
But generally speaking yes, there's cash or cash-equivalent in the overall market's mix, and it stays relatively constant. When these cash assets get dumped onto balance sheets, other assets get bid up until their sizes are appropriate for the amount of cash around.
Index funds do not have opinions on the attractiveness of prices. What happens in this hypothetical when one index fund has a cash inflow is that it bids on all the assets it is "short" of, which increases the price until the other index funds have "too much" of the now-higher-priced asset and decide to sell.
But generally speaking yes, there's cash or cash-equivalent in the overall market's mix, and it stays relatively constant. When these cash assets get dumped onto balance sheets, other assets get bid up until their sizes are appropriate for the amount of cash around.