If you look in UnityCsReference/Editor/Mono/UIElements/Controls at master · Unity-Technologies/UnityCsReference · GitHub, you can see entries for things like the BoundsField or the ColorField. But if you look for some other things, like the UnityEditor.UIElements.Vector2Field, it’s not there.
Am I missing something? Or why is the source-code for Vector2Field (among many others) not available, but other things in the same namespace are?