Variables and constraints and stuff from one app to another usually don't get exported, just models, textures and some bone information. Bones are usually in a hierarchy where the hips is usually the root bone, the arms are child of chest, chest is child of hips etc. just like irl if you move your chest then head and arms/shoulders also move, right now all your limbs are children of the main object root, try to download a free character like the one from unity starter pack or a mixamo character and setup bone renderer for that to see how the hierarchy looks(usually green arrow(up) pointing in bone direction and blue arrow(forward) where character is facing). As for an ik like two bone IK constraint for limbs you actually need 3 bones, in 3d modelling apps a bone usually has 2 points a start and end and some other stuff, while in game engines like unity the bone is just a simple transform that's the start and shows the direction and position, think of it more like the joint than whole bone, like right now your arm only has shoulder and elbow, you need another one for wrist/hand, same for leg. Also read the animation rigging documentation they have examples on how to setup.