Just go and look at the source code and you will know.

export const mapMutations = normalizeNamespace((namespace, mutations) => {
  const res = {}
  normalizeMap(mutations).forEach(({ key, val }) => {
    val = namespace + val
    res[key] = function mappedMutation (...args) {
      if (namespace && !getModuleByNamespace(this.$store, 'mapMutations', namespace)) {
        return
      }
      
      // 在这里调用了commit方法
      return this.$store.commit.apply(this.$store, [val].concat(args))
    }
  })
  return res
})

The following is the definition of the commit method

this.commit = function boundCommit (type, payload, options) {
    // store 就是你想要的答案
    return commit.call(store, type, payload, options)
}

this.setCart() is mapped to this.$store.commit('setCart')