Physically, developers cannot fill the game with "everything at once", so they often give this opportunity into the hands of users. Then games quickly begin to grow overgrown with add-ons or "mods" that easily connect to the original.
Instructions
Step 1
Official mods connect easily. They are specially designed to require a minimum of effort from the user who wants to install them. If this is a DLC or a regular Add-on, then you will need to do nothing more than launch the installer that will install it. If this is a "standalone" add-on, then you don't even need the original game on your hard drive (eg "Contract J. A. C. K." does not require "No One Lives Forever 2").
Step 2
Check for the "add-ons" button in the main menu. If it is there, then the installation of mods should not be difficult: you only need to place the modification files in the root directory of the game, or a certain folder in it (for each case - your own). Then - start the game, select the "add-ons" item, and in it - the recently installed one. This method is typical mainly for RPGs, but shooters also come across (Doom 3, for example).
Step 3
Check for an add-on launcher. If a menu appears before starting the game, and the "add-ons" item is in it, then this allows you to use several modifications at once. This is typical for Bethesda games like Oblivion and Fallout 3. Having placed several modifications in the game folder, you can arbitrarily connect and disconnect any of them. Thus, many gamers completely change the look of the game by installing several dozen modifications at the same time.
Step 4
To replace models and sounds - replace files. This is the most "barbaric" way of installing mods and is used mainly for older games such as Counter-Strike 1.6. Installation is as follows: you download some files, open the root directory of the game, find similar names and replace the existing ones with new ones. It is worth noting that if the modification does not work well, then you will not be able to return everything back, so be sure to make a "backup" of the original game files.
