How can I apply a simple XOR encryption/decryption mechanism in Delphi with no third party packages?

How can I apply a file encryption/decryption mechanism correctly in Delphi?

I found a very old source that supposedly teaches how to do just that, but using strings. After my previous question here, that doesn't appear to be the correct approach. Rather I should perform byte operations. This is what my old source proposes:

1-) Read the file using the old binded C function:

function ReadFile(path: String): AnsiString;
var
  file: File;
  type: Byte;
begin
  type:= FileMode;
  try
    FileMode := 0;
    AssignFile(file, path);
{$I-}
    Reset(file, 1);
{$I+}
    if IOResult = 0 then
      try
        SetLength(Result, FileSize(file));

        if Length(Result) > 0 then

        begin
{$I-}
          BlockRead(file, Result[1], Length(Result));
{$I+}
          if IOResult <> 0 then
            Result := '';
        end;
      finally
        CloseFile(file);
      end;
  finally
    FileMode := type;
  end;
end;

2-) Apply the XOR operations character by character on this huge AnsiString, like so:

 function Xor(Buffer: String; Key: integer): String;
  var
    i, c, x: integer;
  begin
    for i := 1 to Length(Buffer) do
    begin
      c := integer(Buffer[i]);
      x := c xor Key;
      Result := Result + Char(x);
    end;
  end;

But the problem afterwards, and is what my source does NOT show, is how to convert this transformed string back to bytes so I can have a functional resource to write back to a new decrypted file.

Apparently this is not the correct approach to encrypt and decrypt a file. Can someone please explain to me the correct approach to properly:

Read a file, apply a basic XOR encryption mechanism and then be able to read this file again, decrypt it and use this VALID binary (DLL, EXE) data to write back into a new decrypted file?