Every employee in Germany is required to pay a share of about 20% of their income (1) into the retirement „fund“ — which isn’t one really as the money is not invested to gain interest but instead is immediately payed out to the current generation of retirees. On the other hand, employees are promised to receive a future pension based on the payments by the next generation of employees (i.e. their kids). In Germany, this is called „Generationenvertrag“ („contract between generations“).
This is almost like a Ponzi/pyramid scheme where obscene „profits“ are payed out purely based on the exponentially growing number of payees until the day the growth stops and the system collapses like a house of cards.
This breakdown has been happening for decades already albeit at a glacial rate in the German public pension system as the population has been shrinking and therefore the number of payees. It is artificially kept alive as the German federal government pumps 100 billions of tax money every year into it which is 30% of the whole federal budget …
Of course, a redistributive pension system can sustain itself as long as the population age distribution looks like a literal pyramid. But that’s not the case for most Western countries.
(1) Formally, the employee pays only 10% and the other 10% are taken from the employer. But, of course, the employer could as well pay his 10% share as salary instead. So, either way, it’s the employee paying everything.