Datasheet view shows the data in the database. It also allows you to enter and edit the data. It does not let you change the format of the database, other than minor changes (such as displayed column widths).

Design view allows you to create or change the table, form, or other database object, and configure the fields. You can also set keys and restrict the values entered here. But you can't change the database data in design view.

You don't use one or the other exclusively. You constantly switch back and forth between them while designing your database. Use the View button to do this.

When any database object is open and has focus, the leftmost button on the standard toolbar is the view button. Use it to toggle between datasheet view and design view.

In design view, clicking the View button switches you to datasheet view.

In datasheet view, clicking the View button switches you to design view.

You can select other views with its dropdown arrow.

It is probably the most used button on the toolbar.

I usually use Create Table in Design View to makle the database table, then enter the data with either the datasheet view or a form.