where do undefined symbols get resolved?

[Doug Evans]

Would someone mind pointing me to the source for the code that
resolves undefined symbols?

target = elf

input = foo.o --just-symbols=foo2.elf

foo.o has an undefined symbol that is resolved by foo2.elf
which is a fully linked executable.

Where in the linker or bfd sources is the code that resolves
the undefined symbol?  i.e. the actual line that says, figuratively,

   undefined_symbol->value = x;