I suppose technically a database is just a CRUD app
For this type of development you want the DB to handle basically all the heavy lifting, the trick is to design the DB schema well and have the web API send the right SQL to get exactly the data you need for any given request and then your app will generally be nice and snappy. 90-99% of the logic is just SQL.
For the C# example you'd typically use Entity Framework so the entirety of the DB schema and the DB interaction etc is defined within the C# code.
A lot of apps are just about putting data into a DB and then providing access to that data. Maybe using the data to draw some nice graphs and stuff. That's a crud app.