Files with very poor quality

The  .psd file coming out from Photoshop 2024 is 458 MB.

Then I save it as .jpeg (save as) and the result is 3 MB!!!!! 

Absolutely nothing wrong here. JPG is compression. It is supposed to reduce the file size without (much) noticeable loss of quality. Your assumption that file size indicates the quality of the image is completely incorrect. To judge the quality of the JPG output, you can simply look at the image (and not look at the file size).

This is normal.

Jpeg uses very aggressive compression to reduce file size for storage and transfer. But there's a price. Every time you resave a jpeg it degrades a bit, and this is irreversible.

The reason jpeg is still used, despite the drawbacks, is that the compression is incredibly effective. It can reduce size to 2-5 % of native size, with acceptable visual quality.