Number one of their income is obviously lending 
it could be lending to other bank like credit suisse case or lent to other people, a company or government project or lend their money to the government bond. the point is they generated money from lending. and that is pretty much 90% i think
Another things is through transactions, you are not giving the service for free. But biggest source indeed is through lending or loan (in my area). It allows the bank to make their funds revolve around their users with monetary interest. It is the nature of centralization which made this possible and if we would compare it from this technology then there's a huge difference with how money flows. As we all know, banks have plenty of users which is considered aspect of bank. In this industry as well but centralization and decentralization are far from one another. As ive read in the replies, banks has the weath and all but due to centralization, they are also liable withnegative tendencies such as losses. it could be lending to other bank like credit suisse case or lent to other people, a company or government project or lend their money to the government bond. the point is they generated money from lending. and that is pretty much 90% i think Right, even in my country lending might be the fuel for banks and it might happen to other developing countries. I also watched some youtube channel that discussed about curent bank collapse the SVB bank and they showed the basic of the bank too - https://www.youtube.com/watch?v=E_VKZ6dzauo