About the "GetBytes" implementation in BitConverter

Could someone provide a more detailed explanation here?

The MSDN documentation for fixed comes with numerous examples and explanation -- if that's not sufficient, then you'll need to clarify which specific part you don't understand.

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And how should I implement this function in standard C++?

#include <cstring>
#include <vector>

std::vector<unsigned char> GetBytes(int value)
{
    std::vector<unsigned char> bytes(sizeof(int));
    std::memcpy(&bytes[0], &value, sizeof(int));
    return bytes;
}