CATIA V5-6R2021 (the version often referred to as "2021") is a high-end CAD suite by Dassault Systèmes. To download and install it officially, you must use the Dassault Systèmes Download Portal. Note that the specific version "R21" is an older release from around 2011, whereas "2021" typically refers to the newer V5-6R2021 release. How to Download & Install CATIA V5 (2021 Version)
Instead of hunting for a dangerous "R21" download, use these official channels: download catia v5 r21 2021
The Moment of Truth
System Requirements: