Vanguard's S&P (Standard & Poors) 500 index fund tracks the S&P 500 index, on which it's based. When two companies in the S&P 500 have a merger, that leaves only 499 companies in the S&P 500 and so Standard & Poors names the new entry on the S&P 500. Also stocks can fall off the bottom of the S&P 500 and get replaced.
In order to thwart people who try to profit off companies being newly included in the S&P 500 index, Vanguard actually buys about 508 stocks rather than 500. That prevents people from buying stock #501 and waiting until Vanguard's huge cash inflow starts buying shares (when #501 moves up to #500 on the S&P 500).
You asked about interest, which for stocks is called "dividends". Different companies issue dividends at different times. Vanguard collects 3 months of dividends at a time, and issues you a quarterly dividend. Note they actually subtract their expense ratio from this money, before giving it to you. That's how the 0.05% or 0.16% of annual expenses get's paid - from company stock dividends.
You actually have a choice of letting dividends become cash, or immediately having Vanguard buy more shares of the S&P 500 index ("reinvest dividends") with them. Vanguard has some presence in UK, but not sure which individual countries in Europe.
If you don't have Vanguard in your country, you might buy ETF shares like "VOO" (Vanguard S&P 500 Index ETF). You'll need a brokerage, and will have to pay to buy and sell shares. But you can invest in Vanguard ETFs without having an account with Vanguard.