Their developers are supposed to be very competent and careful, but mostly because of culture and the application of development processes that consider lots of potential errors. The default assurance guidance documents (don't call them standards, for rather pedantic reasons) are ED-79 (for Europe because we're taking about Airbus, jointly published as ARP4754 in the US) for aircraft and system design, ARP4761/ED-135 for the accompanying safety analyses, DO-178/ED-12 for software development and DO-254/ED-80 for hardware development. DO-254 gets augmented by AC 20-152A to clarify a number of points. Regulators who certify the system or aircraft also have guidance about what level of involvement they should have in the development process, based on lots of factors, but with most of them boiling down to prior experience of the developers.
You can read online about the objectives in those documents, but flight control systems have potentially catastrophic failure effects, so they need to be developed to DAL A. For transport category aircraft, per AC 25.1309-1B, a catastrophic effect should occur no more often than once per billion operational hours. Catastrophic effects must not result from any single failure; there must be redundancy in the aircraft or system. Normally, the fault tree analysis can only ignore an event if it's two or three orders of magnitude less likely than the overall objective.
Cosmic rays normally cause more than one single-event upset per 10 trillion hours of operation, so normally there should be hardware and software mechanisms to avoid effects from them. In hardware, it might be ECC plus redundant processors with a voting mechanism. For software, it might be what DO-178 calls multiple version dissimilar software independence.
I don't know Airbus itself, and one always has the chance of something like the Boeing 737 MAX MCAS. But typically, companies and regulators do expect these systems to be extremely reliable because the developers are professional and honest: not necessarily super-competent, but super-careful about applying good development practices, having independence in development processes as well as the product, and checking their work with process and quality assurance teams who know what to look for and what to expect.