ELI5: How do they know what's in the vaccines and stuff?

I can't even make an edible cake! How do they know their vaccines work and what's in it? It's just mindblowing lol.<br><br>Edit: wow I forgot about this post i made until now lol. Thank you for all the 1 million comments I woke up to. I'm only seeing the top 10 but you guys are all awesome! I also didn't expect this to get an equal amount of awards and downvotes lol. Love you all! <3