Pretty much all Apple Watch models have at least some nickel in them. Apple doesn't detail the exact chemical composition, but they do say this, "Apple Watch models with a stainless steel or an aluminum case, the stainless steel band release buttons of Apple Watch Series 4 and later and Apple Watch SE, the stainless steel portions of some Apple Watch bands, the metallic portions of the Hermès bands, and the magnets in the watch and bands, each contain some nickel. However, they all fall below the strict nickel restrictions set by European REACH regulation. Therefore, while nickel exposure is unlikely to be a problem, you should be aware of the possibility in case you're susceptible to nickel-related reactions."
You can learn more about the materials in Apple Watch here: https://support.apple.com/en-us/HT204665