Your tree is a Chinese elm. It appears to be in good health overall and is closing the recent pruning wounds fairly well. One issue with the tree is that the multiple trunks or competing branches have very narrow angles. These won't improve with time but instead will grow pressing against each other preventing a strong union. The hollow area is going to have some considerable decay inside and there is not much you can do about that. We don't normally recommend filling tree cavities but you may want to try using one of the aerosol foams that are used to fill in and insulate around doors and windows. Take care not to overapply as they expand after application. Once it is dry use a knife to remove excess foam down to just below the level of the bark. The tree may over time close over this area. Filling the cavity won't make the rot go away or strengthen the branch attachment but it may help deter standing water in the tree which can be a mosquito breeding ground and it can also help deter other insects such as carpenter ants from moving in.