IW4x (and the base game) are configured using DVars, or Dynamic Variables.
DVars can be set using the console or configuration files (.cfg).
Some DVars can be set in the game's options menu, but most of them can only be set in the console. Many DVars are restricted to the server or client, some will also depend on other DVars like sv_cheats being set to a specific value.

Console

To open the console, press ~ (tilde) on your keyboard. This key may differ by layout, its usually the key under esc, next to 1.

The IW4x console supports autocompletion, so you can type the beginning of a DVar name and press tab to autocomplete it.
You can expand the whole console using shift + ~.

Changing the value of a DVar/executing a command is as simple as typing it, followed by its value and pressing enter.

com_maxfps 333{ENTER}

If you want to set the value of a new/undefined DVar, you can do so using the set command:

set cg_coolDvar 1

Configuration Files

IW4x automatically loads players/iw4x_config.cfg, which is the default config file.

Server configs are stored in the userraw folder instead, they can be named however you like. Pre-made configs are available on GitHub. On servers userraw/autoexec.cfg or mods/modname/autoexec.cfg will also be executed automatically.

Config files can be loaded using the exec command.

exec my_custom_config.cfg

Commonly DVars/Commands