why can’t I make it ice ball?

As an aside, the last 5e character I played before switching was an Order of Scribes wizard, which can do exactly that.

So I regards to the difference between fireball and firebolt, you’d start off casting firebolt and you can increase it’s power until you may as well just spend that energy on the explosive attribute. Though if you didn’t want that explosive attribute you still could cast firebolt with the same energy as fireball.

You can’t though, Firebolt is a cantrip, you can’t upcast cantrips, they just get more powerful as you level up. They don’t even use spell slots, it’s just a different empowering mechanic entirely. Guess what though, what you’ve described is how it works in GURPS.