Some of its developers have been using it by default for some time now, and now all the rest can too. In the second half of 2023, KDE will upload its software to the six, those of Plasma 6, Frameworks 6 and Qt6, but whoever wants to test everything they will present can do so. For this, the best way is to download Kde neon in its Unstable edition and use it in a virtual machine or on a flash drive.
Needless to say, it can also be used as a primary option, but in the state it's in, it's best not to. unless the intention is to collaborate with KDE to get things ready. At the moment, the sixes that are being used are not such, at least two out of three. You are using Qt 6.5.0, but the Frameworks and Plasma have different numbering. In general, when Plasma releases a beta, its numbering is the same as the most recent stable ending in .90, something similar to what happens with Frameworks, which is currently around 5.106 and what appears in the system information is 5.240 . In both cases we would be facing something in alpha or pre-alpha phase, the latter, I would say, since Plasma is at 5.27.80 (it does not reach .90 of the beta).
KDE neon is KDE's own system
KDE neon is the KDE's own system. Although part of his team also works for Kubuntu, neon is what gives them the most freedom to work. They offer the stable version, the Testing, which is like a beta, the Unstable and the developer version. It is in the "unstable" that you can already test the preliminary versions of Plasma and Frameworks 6.
Still, availability has been announced less than a day ago, and many of the news that advanced Are not avaliables. For example, panel is not floating by default, and double click to run doesn't either (one launches). Those who are interested in trying it can download the ISO from the download page, more specifically by clicking on this link. I recommend doing the tests in a virtual machine in programs like GNOME Boxes or VirtualBox, installing the operating system on USB at most. Problems will appear for sure.
The stable version of Plasma 6 and Frameworks 6 will arrive after the summer.