There is a lot to this topic…
1) When you prepare the file for the printing there is usually an option for the printer to print the object as hollow without any preparation of model geometry. But you can of course prepare the model as hollow using extrude to add wall thicknes, boolean some insides of the model and so on... You have to know if it is posible to do so without printing supports, Or you wil have some material in your hollow model.
2)Any model you do is somehow printable. Its good to have it as one object as the printer can have some issues with intersecting parts of different object. When the model is complicated and I use more object then I export it to OBJ and use Meshmixer, a free program with cool ability "make solid" which joints all the objects into one mesh then export to STL and I have STL model ready for printing.
If you dont know anything about printing, try to do some research.
