The founding
Zeno came to Athens from Citium on Cyprus, by tradition after a shipwreck, studied with the Cynic Crates and others, and began teaching in the Stoa Poikile, the Painted Stoa on the north side of the agora. His followers were called after the building: the people of the stoa.
That naming is worth pausing on. The school is named for a public colonnade, not for a doctrine or a founder — a fact about where philosophy was done in Athens as much as about what was taught.
The doctrine, and the evidence problem
Nothing of Zeno's own writing survives complete. Stoicism is reconstructed from fragments, from hostile summaries, and from the Roman Stoics — Seneca, Epictetus, Marcus Aurelius — writing centuries later and with different emphases.
That means the ordinary reader's Stoicism is a Roman-period ethics of self-command, while the early Stoa was a comprehensive system with a physics and a logic that the popular reception ignores entirely. The distinction is not pedantic: the ethics only makes its own kind of sense inside the physics.
Why he appears in the architecture layer
Because the school is named after a building type this platform covers, and because the connection is not incidental. A stoa provided shade, shelter and a place to stand about talking without paying for it, which is why teaching happened there.