Place embedded resolution query

A document has only one resolution.  So if you create a new Photoshop Document with a 300DPI Print resolution all the layer in that document will have a 300DPI Print resolution.  Photoshop is a pixels editor.  Print Resolution is just a setting that set the size a document pixels will be printed.  There are severs ways to get images into your new document,  from others documents and files.  You cans copy and paste between documents, drag layers between documents and duplicate a layer into a different document.  In all these cases pixels are duplicate from one document to the other document.   Pixels are just duplicated into the documents,  the size those pixel will print depends on which document the pixels are in.  A image can be printed any size by changing the Print resolution the printed pixel size.

Now there is Photoshop's  "PLACE" it a strange beast.  Be default Place will create Smart object layers,    Photoshop will use the Image file composite Image' s pixels to create a hardened object.  That is just like a copy past or duplicate document images, The hardened object pixels can not be changed byt Photoshop tools.  However the hardened object is not the smart object layer pixels.  For me there is a big issues in Place.  If the Document the file is being place in has a different print resolution then the file being place in print resolution Place will scale the object degrade the image quality. If the scaled image does not fit within the document canvas Place may degrade the image a second time depending on you photoshop preferences resize during place.    The good news is the object has you image's actual pixels  your image quality can be recovered.  The Bad news is there are other Photoshop preferences for Place.

If you check Skip Transform when Placing there will be no interactive transform during Place. Photoshop will just do the rest of place. If you un-check Resize during Place the only resizing that will not be done is with layers larger the canvas size will not be scaled to fit on canvas. If you un-check Always Create Smart Object when Placing. There will be no hardened object to recovet quality.  You will not be able recover the degraded image lost quality.  Additional scaling will degrads even more.

Still Smart object layer are great. You just need to be careful with document print resolution.    IMO Photoshop scaling based on different print resolution is bogus. The User will scale the image in places interactive transform during place if the image needs to be scaled,  

Photoshop can have only one resolution per file. The resolution of your photoshop file is 300ppi. Therefore the jpg will be upsampled to 300 ppi when it is placed into the 300ppi image.

Smart objects honor the physical print size. That defines a smart object and overrides everything else. This is for compatibility with vector applications like Illustrator.

This is a bit counter-intuitive for Photoshop users who are used to thinking in pixels, and it is non-native behavior for pixel-based images.

In any case, what it means in practice is what you observe: the lower-resolution image gets upsampled. When rasterized, the smart object "parent" resolution overrides all.