First, you need to download the patcher tool itself. Go to the official project page on a modern, working Mac. The tool is open-source and completely free.
According to the developer's documentation, the macOS Catalina Patcher is compatible with the following unsupported Mac models, generally released from 2008 to 2011: macos catalina 1015 19a583 mac app store dow patched
Finding and installing specific older builds of macOS can be highly challenging. Apple regularly updates its servers, deprecates older download links, and rotates security certificates. This makes it difficult to get exact early versions like macOS Catalina 10.15 (Build 19A583). First, you need to download the patcher tool itself
A model from 2008 to 2011 (as Catalina natively supports mid-2012 and newer). deprecates older download links