How to make multiple dropdown all values already selected ?

Then you need to assign all the possible values to the value parameter.

And in case you want to set default values to rule inputs, just type "default" into the search field in the documentation. In the popup you will find some suggestions.

pre-populate the variable(local/ri) with values

a!localVariables(
  local!choices:{a!map(id:1,value:"first"),
  a!map(id:2,value:"second"),
  a!map(id:3,value:"third")
  },
  /*pre-populate the variable with values*/
  local!selectedValue: local!choices.id,
  {
  a!multipleDropdownField(
    label: "Multiple Dropdown",
    labelPosition: "ABOVE",
    choiceLabels:local!choices.value,
    choiceValues: local!choices.id,
    value: local!selectedValue,
    saveInto: {local!selectedValue},
    searchDisplay: "AUTO",
    validations: {}
  )
}
)

Existing thread - community.appian.com/.../to-display-the-check-box-with-all-the-selection-enabled-then-the-user-unselects-few-option-and-submit-it