Depends on the index. The usual ones are indeed market cap weighted, and so adopt the overvaluation of bubble stocks, but there are indexes which are weighted otherwise, in an attempt to avoid that. One example is the RAFI fundamental family of indexes:

https://www.rafi.com/index-strategies/rafi-fundamental-indic...

They are pretty cagey about the exact formula, but they do say that

> Security weights are determined by using fundamental measures of company size (adjusted sales, cash flow, dividends + buybacks, and book value) rather than price (market cap).

The top ten holdings in their US index are (rank - company - weight):

  1 Apple 4.1
  2 Microsoft 3.4
  3 Alphabet 3.3
  4 Berkshire Hathaway 2.3
  5 Amazon 2.2
  6 Meta Platforms 2.2
  7 JPMorgan Chase 2.1
  8 Exxon Mobil 2.0
  9 Bank Of America 1.4
  10 Chevron 1.3

Whereas those of their benchmark, the Solactive GBS United States Large & Mid Cap Index, whatever that is, are:

  1 Nvidia 7.1
  2 Microsoft 7.0
  3 Apple 5.7
  4 Amazon 4.0
  5 Alphabet 3.7
  6 Meta Platforms 3.1
  7 Broadcom 2.4
  8 Tesla 1.7
  9 JPMorgan Chase 1.5
  10 Eli Lilly 1.3