What is the ruling on working with a diploma in civil engineering for construction companies that build various structures, knowing that some of them may build banks or insurance companies?

There is no harm in benefiting from the aforementioned diploma. As for working in construction companies that accept permissible work, there is no objection to it. However, if the company accepts impermissible work – such as building a headquarters for an usurious bank – then one is not permitted to work in that; due to the prohibition of cooperating in sin. Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."