Would like to understand something about IAT PE-format

See more:

C

Windows

Win32

C++11

portable

basically i understand what are unions they share same memory and whatever happens to 1 member it effects the others and vice versa.... now am trying to understand this part of code :

struct _IMAGE_THUNK_DATA64 {
 union { 
  ULONGLONG ForwarderString; // PBYTE
  ULONGLONG Function; // PDWORD 
  ULONGLONG Ordinal; 
  ULONGLONG AddressOfData; // PIMAGE_IMPORT_BY_NAME 
      } u1; 
} IMAGE_THUNK_DATA64; typedef IMAGE_THUNK_DATA64 * PIMAGE_THUNK_DATA64;

am trying to read/parse a process Import address table now what am doing is basically this
:

if (!memory::Read(process_base_address + import_desc.FirstThunk,&first_thunk,sizeof(first_thunk))) return 0;

am reading the data from firstThunk and storing it inside my IMAGE_THUNK_DATA which has the size of 8 bytes since its using a union now the real question is

- when i do this read memory operation which variable exactly am i reading is it the last one that got written to assuming from my target process?

- how would i read for example Ordinal from that union considring its the same value as Function which is wrong

- lastly i found some people doing this

auto function_address = process_base_address + import_desc.FirstThunk + func_index * sizeof(std::uintptr_t); 

where function index is equal to 0,1,2 etc.. and by doing this without reading this line from memory i get access to an array i guess of function pointers that resides in the [.rdata] section and if i try to read it from memory using ReadprocessMemory i get the address of actual function inside the [.text] section and by multiplying it by 8 bytes i go to the next function pointer in memory.

i hope that someone could clarify this since i have been trying to understand it since 3 days and still don't understand how its supposed to work.

What I have tried:

i have done what i posted in my question paragraph but i would like to understand it more.