To create a subpage in Google Sites, you can
Move a page under another page to create a subpage. On a computer, open a site in new Google Sites. At the right, click Pages. Drag the page where you want to move it. To publish your changes, at the top right, click Publish.
Create a new subpage. To create a new subpage, click Pages. Then, choose a page and click More Add subpage.
Use the more, three-dot button to create a subpage.
Click and drag a page to move up and down the hierarchy.
Use the keyboard shortcuts to move pages: use the Ctrl+/ (Cmd+/ on a Mac) to see the keyboard shortcuts.