Ask and you shall receive :
- Open Steam and right click on Humankind in your Steam Library. Go to Manage->Browse Local files.
- You will find the poster in the folder Poster. There is a separate file for each supported language. On the first page of the file you will find the Units tree, while on the second - the Technology tree.
- The soundtrack is available in the folder Soundtrack.

If you do not see those two folders there must be an issue with your purchase. :)

The Posters and Soundtrack are not in that location on my installation. I tried reinstalling and it did not fix the situation.

It may be interesting to note that i purchased humankind and then later purchased the deluxe edition upgrade, perhaps this is a bug with how that upgrade is implemented? i have victor and the notre dame content in game.