If you drag the divider between the message list and preview all the way to the right, the preview will be hidden. You can also drag the vertical dividers that separate the three columns left or right to adjust the widths of the three columns. In Catalina, you can show and hide the right-hand column by choosing Show Side Preview from the View menu.
You can come close to emulating the classic layout in Catalina by choosing some options in the View menu, but by default, the Mail app presents a three-column view: mailboxes on the left, a message list for the selected mailbox in the middle, and a preview of a selected mail message the right.
