YS: In general, I selected a song to fit each scene or location. For example, battle music for a battle scene. However, there are a lot of songs, so they're divided into types: Battle music, Exploration music, Flashback music, Sorrowful music, Calm music and so on. Then I would basically choose a song from within one of those categories.
And for something like exploration, it's something that takes time. Depending on the player, you might spend an hour or two hours exploring, meaning you'll be listening to the same song the whole time. So if it's an intense song, or one that stands out, you'll get sick of it. For exploration music, I choose songs that are mild and non-intense, which are suitable for listening to repeatedly. Songs that can be heard for a long period of time without growing tired of them.
For battles, it has to suit the rush of adrenaline as you charge forward, so battles use songs that are a bit stronger, with a faster tempo.