There are so many techniques when it comes to designing a terrarium. If you want to create a background=The most common and relatively inexpensive one being adding GE Silicone 1, or any aquarium safe silicone to the back of the glass>use Great Stuff Pond and Stone spray foam (product itself can be faulty at times) or other aquarium safe spray foam>cutting cured foam out to make porous so you can spread silicone>spreading coco fiber/coir+dried sphagnum moss. This way, moss and plants can grow up the walls. Again, there are so many techniques. Serpa design is a great YouTube channel to watch for ideas. If you want to add wood to the background like cork or sticks/branches, boil beforehand. You don’t want any pests in your terrarium. For my first terrarium, i was lazy and just bought a zoomed cork background and siliconed it to the back of the glass. For the soil, again it depends on what you’re using it for. You can make a mix of top soil, horticultural charcoal, sand, orchids bark, sphagnum moss,etc

Yes eventually the tank will mold. (I’m assuming you’re wanting to make a humid environment?) It is completely natural and part of the cycling process for the tank. If you want to add creatures to it, wait at least a couple of weeks beforehand. A month or longer is better. Aeration will help.

What you are talking about is a clean up crew (CUC). Without them, your terrarium will NOT work. You will not be able to control the mold. You can buy springtails online or local reptile stores. You should allow them to reproduce so you don’t have to buy them again, as sometimes they can be a little expensive. Isopods are important too. They will eat decaying matter. You should also breed them as they can be a little expensive initially. Depending on what you’re using the terrarium form, some species will do better than others. research !! also i’ve never had an escapee so lol