I am afraid goji (Lycium barbarum) is not a tree but a huge sprawling semi-vining thorny shrub which can reach 15 feet tall/wide. I have never heard of a dwarf form.
I think trying to grow one in an appartment is doomed.

Question 2 - some plants are small by nature but most dwarf fruit trees are bred that way or grafted onto rootstocks which are bred that way. That is not to say they are 'genetically modified' in the sense I think you mean. They have just been selected over generations for certain characteristics in the same way dog breeds were developed.

Here is a link that might be useful: Goji info.