If the game has no anti-piracy protection - then it is exactly how it works: just copying files is enough.

If there is a protection - the game will try to check if you bought the game. However, the game runs on your computer, for which you have absolute control over! Yet.

• If the game checks DVD disk, you can install a fake DVD driver. It will pretend, that there are a disk installed and will allow a program to read it - but will actually read data from a file. Then just copy data from the real DVD to a file.
• You can inspect the program's memory to figure out what its doing, and then edit the EXE to change its behavior. Yes, it contains the dreaded "machine code", the language of the CPU - but it is possible to just learn it (it's description is published on the Internet). You can also use helpful tools, that will convert it into a more human-readable form. The goal is to find a place where the game does a check, and replace it with "always pass".