How does CSPX reinvest dividend?

Dividend reinvestment is done by the fund to puchase more shares making up the fund, i.e. NAV of CSPX increases. This increases the value of each each unit of CSPX but you do not get any extra units of CSPX. Dividend WHT of 15% still applies before dividends are distributed to the fund, and is at 15% (instead of 30%) due to the tax treaty between US and Ireland (since CSPX is Irish domiciled).

Betterspider wrote a good article on how WHT for CSPX works, have a read: https://betterspider.com/tax-efficient-etf-avoi...​​​

CSPX reinvest in shares of the holdings. Since you dont receive dividends, there isnt a 15% tax (irsh-domiciled from London).