KDE releases numbers can be 4 part. The first 3 are for scheduled releases and the 4th one is for unscheduled one. E.h. 5.18.4 was released without having the version number in the source code updated, so 5.18.4.1 was released right after with that fixed. Otherwise FreeBSD has both the Plasma and the Application version number in their meta package. 5.18.4.1 is Plasma and 19.12.3 are applications.
The last three parts are probably a date in yy.m.d format.
Package names only get longer as you add more layers of maintainers. (For example, package-XubuntuY or package+YYYYmmdd-n was common on Ubuntu, who needed their versions to fit "between" given Debian versions.)
https://en.wikipedia.org/wiki/KDE_Software_Compilation