XP/Windows 10: User\My Games\FarmingSimulator19\Mods If a zip file contains the mod itself then the zip file needs to be copied into the mods folder of the game. If there are zip or exe files then these will need to be extracted and exe files run.
Check to see what files are contained within them. RAR files are also used for the map downloads and will usually require extracting to your data folder. You will usually end up with either exe files or zip files. When the download finishes you need to extract all the files within the rar file. Once the file has downloaded just double click on it and when it runs it will install itself, done, run the game and test it.
When you download a mod file it will usually come in one of three file types. How to install Farming Simulator 2019 / 19 Mods on PC