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Here is a collection of articles that will help you: https://www.gardeningknowhow.com/houseplants/bonsai

You may plant these into your bonsai container at this time. This collection of articles will help: https://www.gardeningknowhow.com/houseplants/bonsai
https://www.gardeningknowhow.com/houseplants/bonsai/first-bonsai.htm

It may just have dropped its leaves due to stress, or it has gone into winter dormancy. Only the tropical and subtropical bonsai should be kept indoors. If it's a deciduous tree or conifer it should be outdoors, unless a severe frost is forecast.

You can scrape off a little of a branch to see if it is green underneath. If it is green, it is still alive.

Here is more information:
https://www.gardeningknowhow.com/houseplants/bonsai/bonsai-basics-starting-your-own-bonsai.htm

https://www.gardeningknowhow.com/houseplants/bonsai/bonsai.htm

A moisture meter might help. Here is an article that will introduce bonsai care, along with offering links to other articles of interest for bonsai culture as well. This can be found here: https://www.gardeningknowhow.com/houseplants/bonsai/first-bonsai.htm

Bonsai are best grown outdoors. It may be too late for this bonsai, but this article will help if you get another:

https://www.gardeningknowhow.com/houseplants/bonsai/first-bonsai.htm

From what I can find out of the substance, it will be an excellent material to start seeds and amend soil with. It is common practice to spread this on fields to absorb any extraneous radiation.