It's pretty much like Sifter said, u can do the same in stick editor but in my opinion its faster with premade sticks, since u dont have to make new figures every new frame.
I made little tut for you how im doing my fire (i opened pivot the first time after like a half year break, i was about to stop few times when i was doing it ._.)
I created the shape with some electriciti sticks i had
Then i tried to make the outlines more fire-looking
More sticks
Some colors
More colors
In this frame i kinda overcolored the fire and it was messy
So had to put some bigger sticks on the outlines
And there it is, i would add more details and stuff but i takes eternity to make one frame, so dont even try to waste ur time pivot and try flash, trust me.