So a video game is just a collection of files which can be copied at will to any computer. An important subset of this files contain executable code, a program. Now in principle any computer could run the same code, but the publishers of video games want to restrict the users of their product to those who paid for it.

One way to do this using physical media, e.g. DVDs, is done by removing important pieces of the code (or encrypting it) which is stored in a normal fashion on the DVD. Then at startup of the game, in order to continue it requires these pieces (or a key for decryption) and reads them from parts of the DVD which is hard to copy exactly, so people can not just create and distribute copies of the DVD. This works because end-user DVD burning equipment has much higher read-fidelity than write-fidelity. A skilled cracker could analyze this process at the program's startup and just patch the missing pieces directly into the program and upload it.

A much better and modern approach is DRM like Denuvo. Obviously the companies who make DRM are quite secretive about its working so we have to rely on the crackers who try to circumvent the protection. It usually works by online-activating the program of the game and fingerprinting it to the end-users environment and hardware. So every user then has a slightly different program installed which at startup or even while playing tries to make sure it is run at the very same computer on which it was activated. A cracker must try to analyze the program, which had a lot of senseless data added (obfuscation) in order to prevent being analyzed. They then have to find all functionality in the program which prevents execution on the wrong user hardware and remove that parts of the program. Finally they can upload a program which runs on any user's hardware without online-activation.