What was the INDOOR filming location of the Xanadu mansion & huge fireplace in Citizen Kane?

Those were all stage sets built for the movie, not real locations. They didn't do much location filming in those days like they do now because they had enormous sound stages and lots to work with. Now that most of the major studios have sold off their properties or downsized theitr operations they film more on location and actually have what are called "location scouts" who do exactly that, scout out locations for filming various scenes. Cheaper to do and they actually make money.