Many but not all and that is the important detail.
Also I keep money in index funds even if I don't plan on using that money for retirement. (e.g. downpayment on a house, saving for another large purchase, financial buffer, etc.) I also keep money in index funds that are in regular brokerage accounts because 401k + backdoor roth ira isn't sufficient for retirement if you make $200k+/yr. (True for even lower amounts too but whatever)
That depends on which country you live in. In my country (The Netherlands) you pay a fixed percentage of the value of your portfolio. Dividends are not taxed.
and you have to care about where the index fund is domiciled. Otherwise you get dividend leakage. Some Vanguards are domiciled in Reland where 30% of dividends are withheld as taxes even though we need to pay just 15%. It might be difficult to get the difference back
Indeed it does. Here in Brazil, for instance, dividends are not taxed, while selling a stock (to take advantage of growing stock prices) has a capital gains tax of 15%.