"Obviously a huge part of an index is maintaining desired weightings" Actually it's not. Suppose I have an index that should contain AAPL and TSLA weighted by market cap, with total value $100,000. Say they both have 1000 shares in existence, and on Monday their prices are $100 and $300 respectively. Then my index will consist of $25,000 = 250 shares in AAPL and $75,000 = 250 shares in TSLA.

Now suppose on Tuesday the price of TSLA has increased to $700, so my index should be 1/8 AAPL and 7/8 TSLA. It's tempting to think this means I have to rebalance by selling AAPL/buying TLSA. But that's not right! I have 250 AAPL shares which are still worth $25,000, and 250 TSLA shares which are now worth $175,000. So I'm already at the desired 1:8 ratio.