Linux is a kernel, but people often refer to the whole thing as linux…
Everything else is independent free software, which without a distribution you would have to source yourself, configure, and install. Plus provide small programs / scripts to glue everything together. This same software can run on other operating systems depending on what it is. Unlike Mac and Windows, these are often by one vendor and highly integrated, less so with other operating systems.
KDE and Gnome, are desktop environments which are suites of applications, including a “window manager” which is the thing which draws borders, and allows you to minimize and maximize. Typically this is what non technical users think an “operating system is”
Distributions are highly varied in terms of the glue, and updates they provide. The idea is they keep up to date on the software and take responsibility (most of the time) for integrating it and ensuring that the configuration works.
Probably because there is no interest in open standards. I find ical is more a file format than a transport format, an actual standard protocol around it would be great. Although I suspect that the “transport” was always supposed to be email.