“The safest way to obtain mods is through the official channels that the community already trusts. Paradox Mods is actively moderated and each mod is verified.” Cities Skylines 2 Wiki · 7 months ago
Downloading mods for Cities: Skylines (CS1) and Cities: Skylines II (CS2) is free and generally safe, provided you use official platforms. Official & Safe Sources
If you tell me which you have (Steam, Epic, or Game Pass), I can give you a specific guide: How to install them for your specific platform