Depending on the purpose of the modification, the way to install mods for Car Mechanic Simulator 2018 may be different.
If you need to install a car mod then:
1) Unzip the archive downloaded archive mod
2) Copy the folder under Car Mechanic Simulator 2018\cms2018_Data\StreamingAssets\Car
Avoid double folders, look carefully to have a folder with the name of the car and its files.
If you need to install another mod:
Most likely the installation will also take place in the root folder of the game (\Steam\SteamApps\common\Car Mechanic Simulator 2018), see the instructions for the mod, if none, see the name of the folders in the archive mod and extract the contents of these folders in similar folders in the root of the game.