Habitable zone is that distance from the star where it’s not too hot and you don’t boil off the water. Not too far out, so it’s not too cold, so all the water gets frozen. That zone around the star where the temperatures are in the range where you can have liquid water and hence, where life might exist.
You will note that within the habitable zone around our sun are Venus, Earth and Mars were all capable of having liquid water.
Venus got too hot, because of the greenhouse effect. Mars is too small, so it may have had it, but it lost its interior heat. And so the only one remaining and it is squarely in the habitable zone is Earth.
There are about 100 billion stars in the Milky Way Galaxy. About 30% of all stars have planets.
Hydrogen of course is the most abundant isotope element in the universe by far.