Hello!
If you have a folder, just put the [entire] folder in "My Documents\SimCity 4\Plugins"
If it is just a file instead of a whole folder, then first create a new folder in "My Documents\SimCity 4\Plugins" and name it yourself, preferably according to the properties of the plug-in [for example, Guangzhou Twin Towers, then name it GZSZT (starting with the first letter), which will make it easier to find it later.] Name it, preferably in English, and then just move all the files to this folder. Don’t move the files separately, so that It will not be displayed in the game...
Through the above method, basically as long as you have downloaded the support file, the plug-in will be displayed in the game and display normally. If not, then it is missing. Supporting documents...