The new created page layouts are stored under
Master Page Gallery > Master pages and page layouts by default.
To use the custom page layout to create new pages, you need to add the Redirect Page content type to the Pages list (not the Site Pages Library) via
List Settings > Content Types > Add from existing site content types > Redirect Page.
Then you will be able to create redirect page with the “Redirect” page layout or your custom page layout.
Note: New redirect pages can only be created in the Pages list.