The Blendshapes plugin exports its files to the C:/Users/Public/Documents/ZBrushData2019/ZPluginData/BlendShapesData folder. It saves out *.MA files for the subtool and blendshapes separately but if you try loading directly into Maya just select the subtool one - the blendshapes should be loaded automatically.
Alternatively you could use the FBX plugin which has a “Layers” option. Use the Import option in Maya to import the file and the blendshapes will come in with the model - there will be a blendshape tab in the Attribute editor.
To export layers as OBJ you would have to do it by hand. Turn on the layer you want and turn all the others off, then export the OBJ.