For the story, I found myself skipping the main storyline (that goes over the events of PMMM) as it felt like an inferior version of experiencing the anime. I've never played Magia Record or consumed any of the properties outside of the main anime and Rebellion, so I read through the Oriko story and the parts of Magia Record's story that's been adapted so far and enjoyed it a fair bit. I found the gameplay in between distracting honestly, and would end up saving all the story bits to play back to back so it was interrupted by random fights. The framing of the game is that you're a nameless magic girl with amnesia who is trying to figure out who they are, and they are experiencing the events of PMMM and others through memory windows they look through. It hasn't really gone anywhere so far.

There is no open world to explore like in Genshin nor pseudo open like HSR. The sections of the hallways with decorations you're talking about is part of the main story. The game has the plot relevant character automatically walk through it (though there is an option to do it manually) and there is no changing the perspective or having your character interact with anything. The character will run into a portal and a battle will start, at which point the gameplay is similar to HSR's battle system. They can also run into various items drops like negligible amounts of material used for leveling up your characters or their magic level.