check out this https://learn.microsoft.com/en-us/cpp/c-runtime-library/crt-library-features?view=msvc-170

first, it says that UCRT is linked dynamically. second, there are 2 more libraries that compose the CRT

so, ill try to guess:

• without entry, builtins (default libs) are on, initialization code injected, everything builds fine.

• no builtins, memcpy() is declared, but UCRT doesnt have memcpy() so linker skips attaching it and fails.

• no builtins, memcpy_s() is implemented in UCRT, linker attaches UCRT that is special default lib that requires initialization so it attaches other libraries (msvcrt, vcruntime) that has memcpy() so the thing, again, builds fine.