Here is what you do to make the key work.
Set a door/gate down, then right click on the door/gate, the properties editor will open. Under AI Automated, it will say "Use Key" in the box next to that, type Key then click apply changes. Now set a key from the collectables menu right in front of the gate so you know where its at. Go into test level, walk up to the gate, you will hear the "pick up" sound and thats when you can press E to open the gate / door. You will also get the prompt when you are close enough. Everything should work.
Now for the button to press to display the keybinds, its F1. I hope ive helped you somewhat. I was new once and had the same issue. To be honest, I still dont know everything
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