Use the Box Fill tool to fill an area with duplicated tiles
Automatically fill an area on the tilemap with selected tile(s) by using the Box Fill tool, which duplicates the selected tile(s) to fill the area. You can also use the Select tool to select tiles and then use the Box Fill tool to fill an area with them.
Select the Box Fill tool from the toolbar, or press its shortcut key U.
Select the tile or several tiles in the Tile Palette window that you want to fill an area on the tilemap with.
In the Scene view, click and drag the Box Fill tool on the tilemap to select the rectangular area you want to fill.
Release the mouse button to have Unity automatically fill the selected area with the selected tile(s).