Dont get too down on yourself. In reality, you saved yourself tons of money and stress. Realize that all the "real bonsai" trees are NOT meant to be kept indoors. Your plant is.

In many cases, I see bonsai is many people's first attempt at caring for any plant. This is like starting on expert difficulty. You're asking about bonsai soil? It's simply put, rocks. Its holds little to no nutrients and when you pour water in it will quickly drain out. If you've never watered or fed a plant before, good luck.

You have a ginseng ficus. Be happy you didnt get the juniper or other common "big box bonsai". Ficus are tropical and can be kept indoors. Some bonsai purist look down on indoor bonsai but ficus have seeming been the most respected indoor trees.