The Catalina update was a little surprising. In the macOS Software Update panel, it appeared as “macOS Catalina Security Update 2026-001 10.15.8 1.69 GB”.
I don’t recall it generally being the case that Security Updates would increment the OS release number, so perhaps it really is a combination of certificate updates and other, unnamed security patches. At 1.69 GB, it certainly could be both, though macOS update sizes can sometimes be surprisingly large in the era of sealed system images.
Edited to add: FWIW, it took around 20 minutes to install the update, which did seem a bit long if it were only updating certificates. Maybe it took a while to calculate checksums on older hardware.