Can we install dx package unit inside building (not on a roof)??

NO,the effective heat transfer of refrigerant is not possible inside the building.

no because the outside dx units had condensor fan to get out the  high temprture air form the condersor unit, so must putting like this units in a roof or beside the building . while the inside units we can putting it inside the building. and concet them both by piping .

there are some units that is recommended to be indoor installed and operated , it has for example that the condenser fan motors be connected to a duct and most of the times the condenser fans are centrifugal type that can with stand the rise of the ESP ( External static pressure )

Sure we can, this will subjected to criteria of the place and the capacity of unit the usual example installation in a mechanical floor, good ventilation must be provide

yes its called (floor package)....put in that package room you should install Small MSU (mini split unit)