Is zakat due on an advance payment from work, with which a car was not purchased, and which was placed in investment certificates whose principal is paid off monthly from the salary? And what is the ruling on the interest accrued from these certificates?

If the loan is interest-bearing (riba-based), then it is a corrupt and forbidden transaction. It must be repaid immediately, along with repentance, and he is not obligated to repay more than the principal amount. However, if the loan is without interest, then there is no harm in taking it, but it is forbidden to deposit it in usurious banks. Zakat is not obligatory on the profits obtained from investment certificates because they are ill-gotten gains (amwal khabitha). Debt prevents the obligation of Zakat on hidden wealth (gold, silver, and what takes their place from paper currency), provided that the debt consumes the nisab or reduces it, and there are no other non-Zakat-eligible funds.