Arabs called a hill as الجبل. At least, it was in Ancient Arabic. Why? Because there was no "what we call a mountain" in Ancient Arabs' surroundings. The hills were big enough for them to be called as الجبل.
We can see that jabal uhud and jabal an-nur (two historic places) in Saudi and they are just hills if we compare them to the mountains in other countries.
Masculine nouns, especially nonhuman ones, have irregular plurals. There are patterns that you'll notice once you learn enough of them, but it takes a while.
Mountain -- جَبَل, Mountains -- جِبال
Brother -- أَخ, Brothers -- إِخْوة
Restaurant -- مَطْعَم, Restaurants -- مَطاعِم
Door -- باب, Doors -- أَبْواب
Thing -- شَيْء, Things -- أَشْياء
Father -- أب, Fathers -- آباء
Feminine nouns are easier, they usually become plural by changing the ة to ات. (University -- جامِعة, Universities -- جامِعات).
And lots of masculine human nouns have regular plurals ending in ون. (Engineer -- مُهَنْدِس, Engineers -- مُهَنْدِسون)
Because hunak and huna are determiners. They're used to mean "there" and "here" in terms of spacial direction. The "there's" in this sentence is for a null subject. For example, "There's a party over there" There's is a null subject There is telling us that a thing (like a party) is in the distance. As such, a sentence like "There's a party over there" would translate to Arabic as "yujid hafla 3la hunak". Hope this helps