When you make a SharePoint 2019 on-premises solution, you can make not only SPFX solutions, but also old-school WSP-solutions. It allows you not only to use SharePoint API, but also to use your own API services. With SharePoint 2019 you can use WCF service. For example, it’s a good way when you develop an application which uses the external database. This works also for SharePoint 2013 and SharePoint 2016.
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