If you’re supplied a JPEG or PNG - and htere are no edits to it - then changing the file format to another for the sake of it - that is opening the image and resaving it to a TIFF or PSD with no edits - you gain absoultely nothing by doing this - except a larger file size.
If you are making edits - then you need to choose the best option for you for the reproduction - whether that is resaving as JPEG or saving as PDF or saving as TIFF or PSD.
You do what the best option is for you.
placed in InDesign
With TIFF and PSD - the vector masks, shapes, and text layers are rasterised to the native file resolution of the TIFF or PSD.
For this reason - if you have vector masks/shapes, text layers in your Photoshop files, then you’re better off saving as a PDF.
And note - any Smart Object or Smart Layer is also rasterised to the native resolution of the Photoshop File.
No, they don’t.
A tiff saved to JPEG with maximum compression - you won’t see any artifacts.
You can test this yourself by taking a Hi Res Tiff - save it as JPEG with maximum compression
Place the JPEG on a layer above in the original image.
Set the JPEG layer to difference - zoom in - any differences will be visible.
Yes, if you save with minimum compression - of course it degrades.
PNG does not have lossy compression. PNG uses LZW compression - which is lossless. Unlike JPG that uses DCT compression.
If you’ve used layers, yes that is handy. If you haven’t - then why bother?
You could do the same with a PDF - TIff - PSD
Or any of the file formats that support layers.