Dpi automatic

If you want 300 ppi, use Image > Image Size, uncheck Resample, type in 300 for Resolution, click OK and follow D. Fosse's directions to keep it 300.

To retain the ppi number, use Save / Save As / Save A Copy.

You have used Export or Save For Web, both of which strip resolution metadata altogether. It does not have a ppi, not 72, not 300, not anything. It's not relevant for screen/web/mobile, so it's stripped.

However, Photoshop needs to have some ppi number when it opens a file, for other reasons. So a default number is assigned, and this just happens to be 72. The actual number doesn't matter; there just has to be one. Other applications will assign 96, e.g. all Microsoft apps.

So that's where the 72 number comes from.