In Visual Studio 2019, "Windows Forms App" template is intended for WinForms applications using .NET 5.0 (or .NET Core 3.1), while "Windows Forms App (.NET Framework)" is for WinForms applications using .NET Framework 4.8 (or 2.0/3.5/4.0-4.6). .NET Framework 4.8 is the last version of .NET Framework (apart from security fixes it won't see any furher development and updates).
1) Right-click somewhere in the Toolbox window, select 'Add Tab', and provide a name. In File Explorer, browse to "c:\Program Files (x86)\Rebex Components R6.5\bin\net-4.6" (assuming a .NET Framework 4.6 project and default Rebex install location), and drag&drop the 'Rebex.Terminal.Control.dll' file to the newly created tab. This will populate the tab with a single TerminalControl (the only Rebex WinForms control).
2) We are not aware of such option. Sorry for the confusion caused by the lack of xxxx.Designer.cs files in our sample projecs - these were originally .NET Framework 1.0 projects created in Visual Studio 2003, which did not use .Designer.cs files yet.
3) That depends. If the application is going to use lot of existing code or libraries written for .NET Framework 4.8 or earlier, then "Windows Forms App (.NET Framework)" might be the best choice for now. If it's a new application developed basically from scratch, then "Windows Forms App" for .NET 6.0 would be a better option. However, in that case, consider upgrading to Visual Studio 2022, because Visual Studio 2019 did not support .NET 6.0 yet (or use .NET 5.0 for now).