Use New Builder (top-right of the editor) opens a drag-and-drop canvas. The left panel offers V2 Blocks (new builder blocks) and V1 Blocks (the classic modules, with the note that "Options on V1 blocks may not fully work on the new builder"), with a Search blocks… box. The centre is the canvas; the floating Inspector on the right has Traits (block options, applied with Apply Changes), Styles and Layers tabs and can be docked/undocked and dragged.

Header buttons: Publish Page (turns the builder output on for the public page), Import (bring the page's existing modules into the canvas as placeholders) and Save Page. The builder is optional: until you press Publish Page the public site keeps showing the classic module version. Turning the toggle on also switches off Live edit, and the toggle label changes to "Use Old Builder" while the canvas is open.