app.config & appsetting.json

In visual c# using-asp.net core web application(.net framework) 

core web using .net fw have a [app.config,appsetting.json] ,so i not sure where to write my connectionstring to make to the db connection?

.so how to use and what are the difference?

Another difference i need to know is the  web.config -app.config-appsetting.json. ?