What forms when carbon-14 decays?
Carbon-14 decays to nitrogen-14.
See https://en.wikipedia.org/wiki/Carbon-14, which states that carbon-14 decays thusly:
This is a typical electron beta decay. The electron,
The above reaction can be reversed, regenerating carbon-14, by the action of cosmic rays on atmospheric nitrogen. In this way carbon-14 maintains a continued existence even after billions of years on Earth, long after it "should have" all decayed.
The concentration of carbon-14 in organic matter can be used to date that matter, up to roughly 50,000 years depending on the method and sample preparation. See the Wikipedia sute referenced above, and also https://www.radiocarbon.com/about-carbon-dating.htm.