banks normally use different systems for their islamic banking transactions... say Doha bank has some people banking with them on the islamic system and others on the non-islmaic .. they have different account numbers, different systems to process their transactions .. and regarding the interest .. islamic banks in case of a car loan for example buys the car and then sells it to you as a customer adding some percentage of profile which is 100% halal that if u buy something and want to sell it to another person u have the right to add some percentage of profit on the original price at which u bought that particular item .. and there are some different names for these processes as Mudaraba, Murabaha, some islamic names for these processes

so they are islamic banking .. and they don't even mix your money with this that contains a percentage of Riba