Thanks a lot for such a thorough review!
Wow, great job! You got the copper/metal/gold nuggets, which is two recipes away from one of the endings. And there is no fire + scientist recipe indeed, the fire is only used to get the nuggets. I guess I made it a bit too complicated for a jam game, as I don't think anyone has reached any endings so far. Which have some background graphics I've put a lot of effort in. Should've put it into the game design instead. =D
Hmm, I used capsule colliders to detect collisions. Usually, it wouldn't detect a combination in case you have more than one object colliding at the same time, so that might be the issue. Wanted to fix it but didn't manage in time. Yep, a grid is a good idea or I was also thinking of a way how play can explicitly connect two items by clicking through them or at least if the game showed some visual feedback that the items are combined. Thanks a lot for pointing out this user experience issue!
Thank you for trying it out and for the kind words!