All of your collected items must be stored somewhere. By default, there are going to your inventory.
But how can you open it?
As in many other games, the default key for inventory is “TAB“. Just press it once. Your inventory should appear on the screen.
If it doesn’t, check the key in the settings.
To do that, press “ESC” and then navigate to “Settings”. Select the “Key Binding” tab, and change the key in the “User Interface” section.