Pre-ordered, no key, no more information, just the closed beta sat in my library?

How do I access the game?!

You need to provide more information - where did you preorder the game and did you check your store profile or email address associated with that profile ?

For example I bought the game in July and activated my Steam key back then. It has been visible for the last 3 weeks in my Steam account.

Where did you pre-order the game? If you bought it directly from Steam, it should be in your library immediately. It's really important that you provide as much information as possible because we're not mind readers and can't give you advice if you're not open with what the problem really is.

The Closed Beta is a separate Entry in Steam. Go to https://store.steampowered.com/app/1124300/HUMANKIND/ and see whether the game is in your library.

If not, then you need to dig up your purchase details/receipt and check where you bought the game and have you received a key from them. Again, if no key is present, you will need to contact that store directly and see what is going on.