I have heard vitamins such as B12 can only be found in meat. Are there any other nutrients as well? Do you have to then take pills for B12 (as an example) to supplement your diet as a vegetarian/vegan?
Even Vitamin B12 can be found outside of meat. It can only be formed by a few bacteria that almost all live in the intestinal flora of animals, but that means you can also find it in other animal products like milk or eggs. You can also find it in some kinds of seaweed but I think this is the only plant that contains it.
So if you are a vegetarian who still eats eggs and consumes dairy products, you still absorb some Vitamin B12. As a vegan that doesn't eat a whole lot of seaweed, I think it is a good idea to supplement your diet with Vitamin B12-supplements. Normally, your body contains a big reserve of Vitamin B12, that lasts longer than a year after you stop absorbing new Vitamin B12, so don't worry if you didn't think of it from the beginning.
About your other question: I think Vitamin B12 is the only nutrient that is almost only present in animal products, but I'm not entirely sure of that.
Nothing. Literally there is no nutrients that can be found in meat which are not in plant based foods. Here is a video from kurgesagt:Why Meat is the Best Worst Thing in the World Meat takes more than 12 hours to digest in gut compared to any other plant based foods it's pretty high, on top of that this much of consumption(literally in millions of tons) of meat is never a thing for a humans of the past, but unfortunately modern human started consuming too much of it so much so that in matter of decades our planet could turn into a brown looking planet from something that looked green due to to much of production of meat.