Should multi-select dropdown/box have an all option?

1. How do I select all options in a dropdown?
2. How do you do a multi select dropdown?
3. When we should allow user to select many options out of given option we should use?

How do I select all options in a dropdown?

To select multiple items, the user has to hold down the shift or ctrl key, then select with the mouse.

How do you do a multi select dropdown?

For a multi select dropdown list category, click + to add items defined in the category to set them as default. You can select one or multiple items up to six as default items. Click DONE to save the selected default items.

When we should allow user to select many options out of given option we should use?

Use a list box when you want to: Enable users to make multiple choices from a set of predefined options.