I have just seen this and I literally spent all the afternoon doing this 




I personally enjoy drawing backgrounds when I can invent them, but it's true that it takes a lot of effort and hours and hours. I've just spent arround 6'21 hours on this, and I still have to paint it




In the end it's just drawing the perspective lines in a first layer and then search for references and more references that can help you finish it. In complex backgrounds like this one here I make different layers so I can draw them without obstacles and then later sort the different elements and make the complete lineart.
But I honestly draw while I watch series or shows, so I'm entertained, because it's very boring to just draw during all this time without doing anything else, I think that's my trick, maybe it helps too?
I'm gonna post a video on how I draw the background on the next days, maybe next week? if you want to see it, you can follow me on instagram
https://www.instagram.com/samiluuofficial
I take reference on pinterest photos but furniture magazine can also help a lot for more casual backgrounds. Just think about your character's personality and the kind on places and enviroments he moves on. In this case, for example, I had to draw the palace of a prince of hell considered the greed demon, and I take influence on ancient egypt to draw him. He also loves cats, so cats are like gods on his country - then I search "egyptian houses", "pyramid", "gold", "bastet statues", "egyptian furniture", "egyptian hotels", etc, etc and I play with gold too to give it the vibes of the demon. I think on how people lives there, what kind of people lives there, what they do,etc.
Sorry for my super-long comment but I think it can all help? 