SharePoint Foundation 2013 migration & upgrade to SharePoint Server 2019
In the migration process when we upgrade the custom solutions from SP 2013 to SP 2019, first we will deploy the custom solutions to SP 2016 and then to 2019. Do we need to upgrade the custom solutions twice to make it compatible for both 2016 and 2019 environments respectively? Or 2016 and 2019 both supports same .Net framework and we will upgrade our solutions only once? Because we would required to open each custom solution source code in Visual Studio and upgrade it for the targeted 2016 or 2019 environment. The custom solutions upgrade and deployment is the only phase where I am confused.
Sample Project Plan for upgrade from SharePoint 2013 to 2019
Are there sample project plans for upgrading SharePoint to 2019, in particular SharePoint 2013 to SharePoint 2019? If not, and if I create one, where would be a good place to store it? SharePoint PnP or some other place? https://github.com/SharePoint/PnP