Isekai wa Smartphone to Tomo ni., Hyakuren no Haou to Seiyaku no Valkyria Recommendations
Both of these animes share a lot of similar overhanging themes and details.
-Person in a strange world
-Smartphone is the only thing still with the Protagonist in this new world
-Protagonist uses a smartphone to create items from their world to progress their new world
-Build a harem, lots o' lolis
-Protagonist is uninterested in his harem and his harem is in love with him
Actually I would NOT recommend any of these anime but if you liked one of them you'll probably like the other one as well. In both anime the average Japanese high school boy is send to an isekai. The twist is that both guys get to keep their smartphones which make the main characters overpowered. The rest of the cast are mostly generic female characters. The difference is that Hyakuren is based on Norse mythology while Isekai wa Smartphone is normal fantasy.
*Gigguk voice* A SMARTPHONE!!!!!!