How to download macOS-Catalina-10.15
Mr. Macintosh lists 8 ways of getting macOS installers. All get them from Apple though some involve running scripts to make it easier. I suggest one of these:
Method 1. The easiest way (if your Mac is eligible) is via the App Store.
Method 8: Direct Apple links to some installers.
Method 5: For myself, I often the Mac Deploy Stick (MDS).  This is 3rd party software (aimed at those who administer many MACs), but inspects what is available from Apple and downloads what you choose.
But select a method for yourself. All download from Apple.