Thank you for the valuable feedback, and being honest on how you felt playing the game. I couldn't make the game I originally wanted to make and I only managed to make a proper win condition and UI the last two hours of the jam, and I made one up quickly as I got really tired from bug fixing. I'm sorry that it felt unclear, but I'll make sure to give more time and love to the game once the jam is over. I originally wanted to make a game on a world where zombies and humans lived in harmony, and we rescue zombies from the human apocalypse. I will think about a score counter, and will add it if it makes sense in the final game, which will be completely different from this submission.
I will take your input on effects feeling arbitrary. I will think on it, and will implement a logical solution to that. Perhaps I could even make a humongous zombie god, that floats in the air, haha.
About the assets, I made the ambulance, and the zombie on the back on Blender, and I did all the code.
Everything else were free assets I downloaded from Itch io and Unity Asset Store. The icons were downloaded from flaticon and their usage terms required me to type in the ahref links in my asset list. I had to rely on external assets as I had only 3 days to make the game.