Can you please explain why we borrow from publicly held banks, despite that there are stockholders who are Jewish?
There are a number of reasons why this is not a problem.
1. Only a minority of stockholders are Jewish. We follow the majority, which is not Jewish, and therefore the bank is not considered Jewish.
2. Shareholding is not considered full ownership. This is disputed among authorities, and Rav Moshe Feinstein ruled that only shareholding with powers of decision-making is considered ownership.
3. Banks are limited corporations and therefore there is no problem of ribbis. This is disputed among authorities but remains a tziruf.
4. Even if there is some Jewish ownership we can say bereirah so that the loan is not from the Jewish part of the bank.