You can’t buy Taylor Swift stock on the exchange. Those publicly known double digit billionaires are that wealthy because they put their company public on the exchange. That company stock is sold at a premium because of high demand. Which means that it’s listed for more than the book value of that company often for 20x more. This is how even founders of companies that have never made a profit become billionaires.
Taylor Swift’s net worth is probably from real dollar profits. If she puts her company trough an IPO she will probably be worth way more than $1.1 billion.