To understand the new query processing in SQL i have created a figure which will let you know the new things added to query processing all the red boxes are the new feature added in sql server 2019.
(1)Here a new dynamic memory grants for rowstore tables, table variable deferred compilation, batch mode on rowstore and more.
(2)And new improvement behind the scence on query optimization that improves quality of the plans for applicable queries.
(3)Cost of adoption is also not much increased.