The Mods menu has a button to "Open Mod Folder", which will attempt to open the folder for you.
The location of the mod folder on your computer is dependent upon which OS you are using and tend to be hidden by the OS. As stated in the Help section of the Mods menu:
Replace USER_NAME with the value appropriate for your computer.Note, if you are using the itch.io launcher to run the game, then your mod folder will be inside the data folder for the launcher.
Windows: C:/Users/USER_NAME/AppData/Roaming/Strive/mods
MacOSX: Users/USER_NAME/Library/Application Support/Strive/mods
Linux: ~/.local/share/Strive/mods
Also, the folder ".../files/scripts/mods/" in the Strive program folder is strictly for the Constants mod that comes with the game.