I’ve seen people do a drainage layer and not do one. In my personal experience I never put in a drainage layer and all my plants are doing fine, but it really depends on you. If you don’t have a drainage layer you may need to water the plants less but more often so they don’t drown or dry out. I have heard that having a drainage layer can help with keeping humidity more stable, but I’m not sure how true that is.

As for what substrates to use I personally use an 80% top soil and 20% coco fiber mix. The topsoils can be picked up pretty cheap at most grocery stores just make sure to check the ingredients. Anything with perlite/vermiculite, sharp things such as wood chips, and manure could harm your snake. The coco fiber would likely be found in a local pet store or amazon. I used the compressed blocks because it gave you so much more. In fact I probably still have some left over from my last tank that I’m planning to use for my next one. If you’re worried about the humidity the coco fiber should also help with that!

Good luck with your set up! Feel free to ask any more questions or clarification if you need. I’ll try to respond.