many thanks @jmcilhinney it is cristal clear.It is possible to use EF Core in .NET Core apps. If you create an app targeting .NET Core and you use EF, you will use EF Core. WinForms was supported in .NET 3.x with C# but not fully. .NET 5 has full support for WinForms so if you create a Windows Forms Application project targeting .NET Core 3.x or .NET 5 then you can use EF Core. Using EF Core is the same no matter the application type. All that differs is how you display the data.
Hi,... and how you store the connection strings...
Thank you @Skydiver it is well understoodYes, that is the .NET Core approach. Now, see how well that works with standard .NET Framework instead of .NET Core.
As a quick reminder, the best practice is not to hard code your settings into the code, but rather put the settings into a settings file like the appsettings.json, app.config, or web.config file. This is where things break down between .NET Framework and .NET Core -- how those settings are stored and how you get to them.