DLL injection using remote thread does nothing at execution - c++

Question

I've been trying to figure out what's wrong for so long.
I've seen some people assign:
GetProcAddress(GetModuleHandle("KERNEL32.dll"), "LoadLibraryA")
And I wonder if that's what I have to do, but I just don't understand what that line of code does exactly. It has nothing to do with MY dll function, so why load it?
Main (console application A.K.A injector):
#include <iostream>
#include <windows.h>
#include <TlHelp32.h>
char* dllPath = "C:\\Users\\Kalist\\Desktop\\Projects\\DLL\\bin\\Debug\\DLL.dll";
typedef DWORD (WINAPI *pThreadFunc)();
char* ProcToInject = "calc.exe";
int main(){
PROCESSENTRY32 pe32;
pe32.dwSize = sizeof(PROCESSENTRY32);
HANDLE procSnap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
DWORD procID;
if(procSnap){
if(Process32First(procSnap, &pe32)){
do{
if(!strcmp(pe32.szExeFile, ProcToInject)){
procID = pe32.th32ProcessID;
break;
}
}while(Process32Next(procSnap, &pe32));
}
CloseHandle(procSnap);
}
HANDLE procAccess = OpenProcess(PROCESS_ALL_ACCESS, false, procID);
void* memSpace = VirtualAllocEx(procAccess, NULL, strlen(dllPath)+1, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
WriteProcessMemory(procAccess, memSpace, dllPath, strlen(dllPath)+1, NULL);
HINSTANCE getLibadd = LoadLibrary(dllPath);
pThreadFunc pThreadFuncVar = (pThreadFunc)GetProcAddress(getLibadd, "threadFunc");
CreateRemoteThread(procAccess, NULL, 0, (LPTHREAD_START_ROUTINE)pThreadFuncVar, memSpace, 0, NULL);
CloseHandle(procAccess);
}
DLL remote process:
#include <iostream>
#include <windows.h>
extern "C" DWORD WINAPI threadFunc(){
MessageBox(0, "Injection worked!", "Injection message", MB_OK);
return 0;
}

Answer 1

The problem with your code is that pThreadFuncVar contains the address of threadFunc in your injector process. However, your Dll.dll is not even loaded in the target process. Even if your dll were loaded, it would likely not be loaded at the same address, so the pThreadFuncVar address would still be meaningless in the target process.
Only a few essentials modules, like KERNEL32, are loaded at the same address in every process. So, if you use the address of LoadLibraryA for CreateRemoteThread, it will load the dll from the path which you copied into the target process's memory. This will in turn call the dll attach procedure of your dll, which is where you want to put the MessageBox call.