Mountains are higher. It's a kind of indefinite thing, the difference between a "pond" (small) and a "lake" (large); or a "town" (small) and a "city" (large). The highest summit within 20 km. of where I live is named "Great Blue Hill," and it is 635 feet or 197 meters in elevation. Mount Greylock, in Western Massachusetts, is called a "mountain," and it is the highest point in the state. It is 3,491 feet or 1,064 meters in elevation. I enjoyed a movie entitled "The Englishman Who Went Up a Hill But Came Down a Mountain." It concerned a Welsh town with a local "mountain." Cartographers come to measure it. In the movie are using a set of standards that says a "mountain" has to be at least 1,000 feet high. I don't know if any such standard ever existed. At any rate, the cartographers measure it and find that it is just under 1,000 feet and is only a "hill." The villagers get together to keep the cartographers in town for long enough for them to carry enough earth to the top of the hill to build it up just high enough to exceed 1,000 feet, and thus qualify it as a "mountain" again.

mountains are higher and made of rocks

They are considered to be different things. A hill may be grassy or sandy with rounded slopes, while mountains are rocky with sharp slopes.

Hills are small mountains.