Flatten image in Photoshop before saving back to LRc

I've got a question!

If I layer two photos in photoshop, do I need to flatten the image or when I export it (I go to file>export as> and save as a high quality jpg) does it automatically flatten the image?  When I open it from my exported location it looks like it does... but not sure if I'm leaving out an important step?

Thank you all for your replies.  Great help and I finally understand x

It might help to know this background info. A layered Photoshop document already provides a flattened version to Lightroom Classic. In Photoshop preferences, when the setting “Maximize PSD and PSB Compatibility” is set to Always, any Photoshop file you save contains an additional flattened composite version of the image. This flattened composite is what lets other applications view and print the Photoshop file, even if those applications don’t work with Photoshop layers. They just read the flattened composite. (This composite does add to the file size of a Photoshop document.)

Lightroom Classic is one of those applications. The Maximize PSD and PSB Compatibility setting must be enabled in Photoshop, or Lightroom Classic refuses to import that file. So if a Photoshop file made it into a Lightroom Classic catalog, you know it has that flattened composite.

Putting it all together: In Lightroom Classic, you catalog a Photoshop file with many layers. Lightroom Classic imports it, but understands only the flattened composite in the Photoshop file. So if you preview, apply Develop edits to, or print that Photoshop file, Lightroom Classic is using that included flattened composite to do those things. Because of all that, you can conclude:

A. You never need to flatten a Photoshop file before importing into Lightroom Classic, because Lightroom Classic can import a layered Photoshop file. For any uses in Lightroom Classic including printing, Lightroom Classic can simply use the flattened composite that’s already included in the layered Photoshop file.

B. In Lightroom Classic, if you like to use the Edit In command to send Photoshop documents back to Photoshop for more editing, then you definitely want to import the original layered Photoshop file, not a flattened version, and also select Edit Original as Per Bentsen shows in his reply above. That will let Lightroom Classic send the original layered file back to Photoshop for editing. If you import a flattened Photoshop file, then when you use Edit In command in Lightroom Classic, only the flattened version would open in Photoshop, which would limit what you can do.

There is no need to flatten files in Photoshop. Lightroom will display the image as if it were flattened.

But the layers will still be there if you want to edit the file in Photoshop again.

You will have to use the option Edit Original, and any edits you have done to the file in LrC will not be visible in PS.

Lightroom doesn't support layers. It doesn't even read them. What it reads is a flattened composite inserted in the file by Photoshop. That's what "maximize compatibility" in Photoshop means.

The jpeg file format doesn't support layers either. Any file has to be flattened before it can be saved to jpeg.

In short - you can rest assured that your file is well and truly flattened and no further flattening will make the slightest difference.