I've recently discovered non-destructive editing in Photoshop. I'm now carefully building my images so that each layer or adjustment layer can be independently edited without destroying the original content. There's just one annoyance, I like to work on the highest resolution 16:9 ration I can but I need to export as a 1080p image. Can I do that without flattening the image and resizing it? This gets rid of all my careful non-destructive work and I can only work around it by saving a copy of my PSD, which is annoying.
You can export the image: File->Export->Export As...
or: "Save for Web" Alt+Shift+Ctrl+S
There you can set the image size and fileformat. Is that what you ask for?
JPEGs can't have layers. The format doesn't support layers. There's no need to flatten the image before exporting as a JPEG. If you want to keep your layers, you will need to save the PSD. The PSD is your original artwork. Open the PSD when you need to output in different formats and sizes.