One problem with carbon dating is that the amount of carbon 14 in the atmosphere is not constant. Fortunately, there are trees that are almost 5000 years old; the varying amounts carbon 14 captured in their rings provides a direct calibration. Humans have used wood for construction for a long, long time, enabling calibrations to go back even further in time. Even older chunks of wood enable calibrations to go back 50000 years.
Another form of radioactive dating is uranium-lead dating. This relies on the very different chemistries of uranium and lead. Lead does not combine chemically nearly as readily as does uranium with other elements. Uranium's chemistry is similar to that of zirconium, enabling uranium to replace zirconium in zircons. Lead is instead chemically rejected when zircons form. This means zircons have essentially no lead content when they form. The lead content in a zircon indicates how long ago the zircon formed.
Other forms of radioactive dating rely on similar concepts. The different chemistries of parent and daughter isotopes is key in many of those techniques.