What is the difference between Dojo framework and Dojo Toolkit?

Dojo has transformed into a framework from 2.0, similar to React.

The Dojo Toolkit has become a pure toolkit.

So which strategy does the latest ArcGIS JS API4.x adopt?

You can see that in addition to using the 1.x version of Dojo Toolkit in Widget, it is divided into AMD mode and ES mode in Build.

https://developers.arcgis.com/javascript/latest/amd-build/

https://developers.arcgis.com/javascript/latest/4.13/