Without getting into details, AFAICT they do not use any typical high-assurance software stacks, such as Ada or Spark or such that might be typical in avionics, like Airbus software. The use off-the-shelf tools like C++ and LLVM.
High assurance comes from the overall systems design and testing, not the software itself. Afterall, radiation can cause any piece of software, including those written in Ada, to completely skip instructions.
A lot of stuff is still going on legacy stacks that Ada was created to replace