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.
In order to enable the WinForms Designer, please get familiar with the requirements you need to meet: Design Time.
The Toolbox version depends on the version of installed NuGet package:
For example, if you have the UI.for.WinForms.AllControls.NetCore and you want to get another version in the Toolbox, it is necessary to:
1. Remove the current version of the UI.for.WinForms.AllControls.NetCore package:
2. Install the desired NuGet version: Install using NuGet Packages
3. Rebuild the project.
The same approach is also valid for the other Telerik UI for WinForms packages, e.g. UI.for.WinForms.AllControls.Net60, UI.for.WinForms.Common, etc.