Populate the choices of a dropdown based on the selection from another

PFB the sample code for your use case

a!localVariables(
  local!state: {
    a!map(id: 1, state: "UP"),
    a!map(id: 2, state: "Maharashtra"),
    a!map(id: 3, state: "Karnataka"),
    a!map(id: 4, state: "Goa")
  },
  local!city: {
    a!map(id: 1, City: "Bangalore", stateId: 3),
    a!map(id: 2, City: "Mumbai", stateId: 2),
    a!map(id: 3, City: "Pune", stateId: 2),
    a!map(id: 4, City: "Vasco da Gama", stateId: 4),
    a!map(id: 5, City: "Lucknow", stateId: 1),
    
  },
  local!selectedStateId,
  local!filterCityBasedOnStateSelected: if(
    a!isNullOrEmpty(local!selectedStateId),
    {},
    index(
      local!city,
      wherecontains(
        tointeger(local!selectedStateId),
        tointeger(index(local!city, "stateId", {}))
      ),
      {}
    )
  ),
  local!selectedCity,
  {
    a!dropdownField(
      label: "State",
      placeholder: "--Select--",
      choiceValues: index(local!state, "id", {}),
      choiceLabels: index(local!state, "state", {}),
      saveInto: {
        local!selectedStateId,
        a!save(local!selectedCity, null)
      },
      value: local!selectedStateId
    ),
    a!dropdownField(
      label: "City",
      placeholder: "--Select--",
      choiceValues: index(
        local!filterCityBasedOnStateSelected,
        "id",
        {}
      ),
      choiceLabels: index(
        local!filterCityBasedOnStateSelected,
        "City",
        {}
      ),
      saveInto: local!selectedCity,
      value: local!selectedCity
    )
  }
)