That's the unexpected turn I'm going for with the farm sim presentation!
Sorry to hear you got stuck on the item system. A few people have been tripped up on that. I ran out of time to implement a controls page in the game, so it's really not clear how it works. I'll add a note to the game page about this.
The inventory can be opened and closed by pressing i on your keyboard. Inventory navigation is done with the arrow keys. If you view the jerry can in your inventory, you can read the recipe again. Combining items is done by selecting two items with interact button (e on keyboard).
Thanks for checking it out and sharing your feedback! :)