Here is a guide on how to acquire the safely.

By following this guide, you bypass the risks and gain a permanent, reusable copy of macOS Catalina 10.15.7—perfect for legacy hardware, software development testing, or simply preserving a version of macOS that refuses to fade away. Download Macos Catalina 10.15.7 Dmg File

Apple distributes the installer as an .app bundle, not a raw DMG. However, you can extract the SharedSupport.dmg as shown above, or create your own DMG using Disk Utility. Here is a guide on how to acquire the safely