This bonsai was bought for me a while ago, and I'm unsure which kind of bonsai it is. My best guess is that it is in the ficus family. I want to know which kind of tree it is partly because it seems to be slowly dying, with a lot of dead leaves present.

Additionally, I don't know how to revive this kind of moss, which died soon after I received the tree.

Any help is appreciated!

Hello and welcome.
Can you post a closeup of the leaves, hard to see on your picture.
Your moss has died because your tree is inside ( but that’s not a problem, that moss is doing no good so you better get rid of it anyway), the tree itself is loosing leaves because it’s inside. Our houses are way to dry, have not enough light and ventilation for a tree ( there is no such thing as an indoor tree). There are of course the (sub) tropical trees that can’t survive out winters, but they are best kept outside untill temps drop in autumn, then brought inside in a cool room ( temp depending on the species), and brought outside again in spring when temps rise

Thank you. I'm able to move it outside now that spring has come, and hopefully that will help bring the tree back to full health. As for the moss, I'll remove it, for it is not helping the tree or its appearance.

First determine the species correct, some tropical trees need temps of above 15, other can stand 5, or even the occasional 0.

I'm thinking its some kind of ficus, although the bark doesn't seem right and the leave seem a bit thin. Here are some more photos, any ideas?

Hey, I think it's a Serissa, at first i thought it's leaves were a bit big for it, but it may be from the lack of light as the trunk maches perfectly with the species, but now with a zoom on the leaves, that's my bet... xD

2nd guess would be pomegranate, but I'm almost sure it's a Serissa...

Thank you! Looking at a few pictures, it does indeed look to be a Serissa.
Many thanks kind sir.