Autodesk AutoCAD Civil 3D 2020 is no longer available as a standard "forever free" download. However, you can access it through official Autodesk channels under specific conditions:
Autodesk offers a 30-day free trial of the software. This is fully functional and legal.
There are two primary legal ways to access the software for free from the official Autodesk website : : Visit the Autodesk Free Trials page and select Civil 3D. Sign in or create a free Autodesk account.
: Use is limited to educational purposes; commercial use is strictly prohibited. 30-Day Free Trial