Libc, clang/gcc, whatever. Needs to be audited.
Perhaps they require the “integrator” to perform the audit or maybe the fact someone provides software which can be useful in critical environments is enough to signal an implied warranty and they are on the hook for compliance. Nobody knows until it all goes through costly legal procedures where everyone is trying to cover their asses and pass the buck.
I’m waiting for the day where FLOSS devs are greeted at EU airports by process servers because they released some software while in college and it got used in some critical software.