There is no investment fund, it is a Ponzi scheme where the current payers cover the current retirees. It works when demography plays in your favour but like any Ponzi once you run out of new entrants the Ponzi collapses it's just a matter of when and how much money printing will be flooded to clean up the mess (same applies to France, Italy...)

Of course. But most of the pensions paid come from a so-called pay-as-you-go system in which employees and employers pay the current pensioners and pension contributions.