reading an application's manifest file?

Windows manifest files are Win32 resources. In other words, they're embedded towards the end of the EXE or DLL. You can use LoadLibraryEx, FindResource, LoadResource and LockResource to load the embedded resource.

Here's a simple example that extracts its own manifest...

BOOL CALLBACK EnumResourceNameCallback(HMODULE hModule, LPCTSTR lpType,
    LPWSTR lpName, LONG_PTR lParam)
{
    HRSRC hResInfo = FindResource(hModule, lpName, lpType);
    DWORD cbResource = SizeofResource(hModule, hResInfo);

    HGLOBAL hResData = LoadResource(hModule, hResInfo);
    const BYTE *pResource = (const BYTE *)LockResource(hResData);

    TCHAR filename[MAX_PATH];
    if (IS_INTRESOURCE(lpName))
        _stprintf_s(filename, _T("#%d.manifest"), lpName);
    else
        _stprintf_s(filename, _T("%s.manifest"), lpName);

    FILE *f = _tfopen(filename, _T("wb"));
    fwrite(pResource, cbResource, 1, f);
    fclose(f);

    UnlockResource(hResData);
    FreeResource(hResData);

    return TRUE;   // Keep going
}

int _tmain(int argc, _TCHAR* argv[])
{
    const TCHAR *pszFileName = argv[0];

    HMODULE hModule = LoadLibraryEx(pszFileName, NULL, LOAD_LIBRARY_AS_DATAFILE);
    EnumResourceNames(hModule, RT_MANIFEST, EnumResourceNameCallback, NULL);
    FreeLibrary(hModule);
    return 0;
}

Alternatively, you can use MT.EXE from the Windows SDK:

>mt -inputresource:dll_with_manifest.dll;#1 -out:extracted.manifest

You can extract/replace/merge/validate manifests using the command line manifest tool, mt.exe, which is part of the Windows SDK:

C:\Program Files\Microsoft SDKs\Windows\v6.1>mt /?
Microsoft (R) Manifest Tool version 5.2.3790.2075
...
> To extract manifest out of a dll:
mt.exe -inputresource:dll_with_manifest.dll;#1 -out:extracted.manifest

Different locations:

• C:\Program Files\Microsoft SDKs\Windows\v6.1\bin
• C:\Program Files (x86)\Windows Kits\10\bin\10.0.18362.0\x86

Open the file in Notepad. The thing's in plain text.