Isopod, millipede, and other detritiovore media serves many important purposes: it provides food, hiding spots, it regulates the humidity, gives a buffer (or even breaks down and recycles) the animal waste products, and in some setups it houses tankmates that help with a lot of our work behind the scenes.
I'll try to break down (pun intented) what usually goes into this substrate, what purpose does it serve, and when/how to use it.
Sphagnum moss
It helps provide a non decomposing "sponge" that prevents humidity swings. The fluffy texture allows for the water to evaporate, thus distributing it as needed. It can also be used on the dry side of the enclosure to provide aeration or help with burrowing.
Compost
Often used as the bulk of the substrate. It's edible, doesn't dry out too fast and can support the microbiome that will prevent ammonia buildup. Some keepers use it as the sole material in the enclosure, and it does work decently for many species, but it will *not* work for everyone.
Note that there are many types of compost, they vary based on location, so you will need to check if the texture is right. Avoid the ones that seem to "mud like", or the ones that clump in unpleasant ways. Look for the ones that seem similar to what you'd find on a forest floor. Also avoid "vermicompost" it's worm poop, and this is for all intensive purposes "spent substrate".
Flake soil
Originally developed for bonsai, it made it's way into the beetle keeping hobby, and them got adopted into the general invertebrate keeping scene. It's hardwood dust/pellets, often supplemented with flour or bran, and processed by yeast through aerobic metabolic processes (not fermentation).
It's a very great accessible and sustainable alternative to white rot wood, as it doesn't require sourcing materials from the wild, and only takes 1-2 months to produce a large quantity. Similarly to white rot wood it is essential for raising certain species. I have seen no reproduction of Desmoxytes planata when they lacked this material.
Wood chips
Helps with aeration. In damper setups they can also decompose, and provide food. In dryer setups (such as for the giant Spanish Porcellio species) it can serve as a good means of making the substrate airy, and provide the mancae tons of hiding spots.
Leaves
One of, if not the most essential staple food source. It's recommended to have 2-6 cm of leaves on top of the substrate. Oak, birch, and magnolia are often used, but many other species work too. Make sure that they are free both from pesticides, and pests. Larger colonies will can devour them quickly, so make sure you have a lot in storage. They also provide hiding spots for the animals.
*Do not allow the leaf liter to be depleted*
White rot wood
As the name suggests it's wood that has rotten and turned white and became easy to chip away at. Nutritionally similar to flake soil, but it comes in larger chunks. For certain purposes it must be ground/blended before use. Most often harvested from the wild.
Coco coir/fibre
Dried fragments from the coconut fruit (not shell). While popular and often instinctively reached for by new keepers, most advanced keepers limit its usage. can be used in small quantities to make the substrate more fluffy. Little to no nutritional value, and certain species of millipedes have been observed to suffer health issues from attempting to consume it.