Autodesk effectively replaced 123D Design with Fusion 360.
Best for beginners. It is a free, web-based tool that works on any Mac browser without installation. Autodesk Fusion
Although 123D Design is no longer available as a standalone application, its legacy lives on in the Autodesk suite of tools. Mac users continue to benefit from the company's commitment to providing accessible, powerful design software. Autodesk 123d Design Download Mac
Autodesk 123D Design was a free, entry-level 3D modeling software suite launched by Autodesk around 2011. It was designed for hobbyists, makers, and novices interested in 3D printing and basic CAD (Computer-Aided Design).
Since 123D Design is effectively obsolete, most Mac users have migrated to these contemporary options: Autodesk effectively replaced 123D Design with Fusion 360
Because Autodesk has removed the official download links from their servers, you cannot download it directly from the source. You must rely on third-party archives.
Security note
Autodesk retired the 123D family (which included 123D Catch, 123D Make, and 123D Design) to consolidate their product lines. They shifted their focus toward (for absolute beginners and education) and Fusion 360 (for professional and prosumer use).