Do not search for “free download Catalina 10.15.7” on random third-party sites. Most of those files are either corrupted, outdated, or contain malware. Only use official Apple sources.
download-catalina-10-15-7-full-installer
Use the official App Store link or the softwareupdate Terminal command. And please—create a bootable USB before you need it. Your future self will thank you.
Whatever the reason, finding a legitimate download link for (the final, most stable version of Catalina) is getting harder. Apple hides these older installers.