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Telerik UI for WinForms controls used to be automatically added to Visual Studio's Toolbox during the installation process when using .NET 2.0 (up to 4.8) Framework in your project. This means that the Toolbox population relies on the Telerik WinForms version available in the Global Assembly Cache.
With the introduction of .NET Core/.NET 5 and the WinForms designer, the Toolbox configuration doesn't rely on the GAC anymore and it is not necessary to have an installation of the Telerik UI for WinForms suite. The Toolbox version depends on the installed NuGet package:
https://docs.telerik.com/devtools/winforms/core/core-toolbox-version
There is a known issue that any UserControls/ custom controls don't show up in the toolbox for a VB project with .NET6. Even the RadControls are not loaded in the toolbox.
This seems to be a known issue introduced in Visual Studio 2022 17.3 only for VB project with .NET6. Microsoft are currently working on the issue and I believe that an update will be introduced with the fix: https://developercommunity.visualstudio.com/t/Custom-and-User-Controls-not-shown-in-To/10118197