You can do it manually. When editing a dashboard, you can tap the ... in the top-right corner of a tile and select the Copy option. You’ll be presented with a list of dashboards that you can copy a tile to. When copied, the tile will retain all of the customizations you’ve applied in the Tile Editor. This would need to be repeated for each tile.
Yes, you can use the URL option in Custom Tiles to embed another page using an iframe. There’s also a Refreshable iframe custom tile available to import. In general we do not recommend embedding dashboards within dashboards using iframes as it effectively runs the ‘app’ twice.