ASP.NET MVC succeeded Web Forms from 2009 onward, separating models, views and controllers, and it still runs a large number of mid-2010s enterprise web applications built before the move to ASP.NET Core.
Signs your ASP.NET MVC system needs a specialist, not a generalist.
Move the application's API layer to ASP.NET Core first, while the existing MVC front end keeps serving users, then migrate the UI once the foundation is solid.
Tell us what's going on — most first conversations take fifteen minutes.