What's the relation of KDE Linux with KDE Neon? The former was made by Blue System and the new one by Techpaladin or something like that?

KDE Neon was originally founded by devs who worked on Kubuntu previously, and some of that team has now moved on to KDE Linux.

The company stuff in the background doesn't really matter.

The team working on KDE Linux are motivated by addressing some structural challenges that always plagued KDE Neon from the concept of trying to graft more recent SW on top of the Ubuntu LTS base, plus some lessons learned from the SteamOS project's way of handling updates, and fully utilizing more recent Linux/systemd features.

It's sufficiently different that sticking with the Neon brand and swapping it out for that userbase would have been pretty disruptive, so they felt it was better to go with a distinct identity.