The Stoics borrowed their arche of the universe from Heraclitus. Heraclitus believed that the universal element of all things was ultimately fire. It’s not clear whether he believed this metaphorically or literally. Heraclitus believed the universe always in a state of becoming, never any substantial being to anything. Everything was in a state of flux.
One can suppose that Heraclitus believed fire was the primary element of the universe because he saw everything always in a state of process or transience, things going in and out of existence. A fire starts, burns, and then extinguishes itself. Similarly things are born, sustain for a little while, and then die.
The Logos, which is the word, law, or order of everything is itself fire and manifestly orders the world. Heraclitus saw the Logos as containing contradiction: day/night, birth/death, winter/summer, love/strife, war/peace, etc, etc. The Logos was the unifying principle of opposites and it explained why people had contradictory opinions. But the Logos was the ultimate truth because it contained all things and all opposites.
The Epicureans borrowed their arche of the universe from Democritus. Democritus believed that all change in the universe was a result of changeless atoms that moved through the void. Democritus had the idea that if you kept cutting things into pieces, you’d eventually yield indivisible pieces that could no longer be cut any further: atoms.
Democritus actually had a pretty good idea about how atoms and the void were real but our impression of sweetness, bitter, cold, hot, color were all in our heads. Democritus believed that only atoms and the void were real but everything else built from atoms was an illusion. This is particularly interesting because it seems almost like a time travelling physicist went back in time and told Democritus that this was really how the universe was ordered.
The Stoics basically took the idea of the Logos and made it into a deterministic driving force of the universe not too dissimilar to Heraclitus. The Epicureans took the idea of the atoms and void and made it into a system of randomness with coincidental order. The Epicureans believed that everything was pretty much randomly produced from atoms and then dispersed back into atoms.
Because of the Stoic’s notion of the universe as deterministic and orderly, they essentially had to believe free will was somehow compatible with determinism. Because of the Epicurean’s notion of the universe as indeterministic and chaotic, they essentially believed free will was totally enabled by atoms being able to randomly swerve and thus people were able to freely do things without being dictated by prior causes.
It’s interesting to think that the Stoic notion of determinism and the Epicurean notion of indeterminism basically foretold 20th century physics. On the macroscopic level, the universe appears to be deterministic. On the quantum level, the universe appears to be indeterministic. It really makes you think that physicists from the 20th century told the Epicureans and Stoics incomplete information about the cosmos and told them that was how things actually were.