Absolutely no loss.

The single greatest 'loss' of a JPEG is when the file is initially created, converting the bit-depth from a typically 12-bit (or higher) from the image sensor into an 8-bit JPEG.

What is 12-bit you might ask? That's 2^12 aka 2 to the power of 12 = 4096. An 8-bit JPEG has 256 tones of each of the three color channels of data (Red, Green, Blue aka RGB). These three RGB color channels work in combination to produce a much subtler, wider range of tones between total black and total white. A 12-bit file can combine RGB into more discrete shades, 4096 specifically for each individual RGB color channel.

When the JPEG is created, that 'shading' of the 12-bit image is mapped to the exact 8-bit color, or the nearest 8-bit value. The human eye can't see all the possible colors of an 8-bit file, so this isn't something you'd notice. The bit-depth enables you to better edit your images, where that wider possible tonal range avoids 'gaps' in the colors, something you can occasional encounter in the blue skies of JPEGs, where the transitions of shades of blue are uneven.

Opening and closing a JPEG does nothing to the contents of the file. Copying / moving the files does nothing to the contents of the file. Only when you 're-fire' the JPEG compression engine to save an edited version of the JPEG does the contents of the JPEG change, possibly. The bit-depth remains the same at 8-bit, so there isn't color data being tossed like the 12-bit to 8-bit conversion. If you change / lower the JPEG quality, only then is more data removed.

We hear about JPEG being a lossy compression. That JPEG 'quality' setting is the lossy compression. Again, that loss occurs when the new image file is created via a digital editor. Opening the file 'inflates' the compressed JPEG as stored in disk. The file doesn't change, viewing the JPEG is simply the software reading the data and 'inflating' the compressed data to display the contents, just like if you unfolded a letter from inside an envelope.

Changing the JPEG quality and saving a new version of the image is like cutting some pieces out of the letter you unfolded. If you cut on the edges, margins of the letter, and the file is smaller but no visual difference to the letter content. Using a still lower quality (say less than 80%) might involve cutting some content, a word or sentence here and there. Maybe it will make a visual difference, maybe not. But, once the trimmed letter is folded back into the envelop, it doesn't change simply from opening and closing over and over again.

I may not know what I'm talking about. What I think happens is that a Save or Save As in Photoshop, or any other editor, causes a JPEG file to be analyzed for compression. So, in theory, there should be some "compression loss". However, in practice, I've never seen visually detectable quality loss through several Saves.

For me and my procedures, it doesn't matter because I use a Lightroom Classic workflow where the original file is left alone. I sometimes force compression in the Export process to reduce file size.

Well, just out of curiosity, I ran some experiments: 4 steps.
1. Started with a Raw file (Z6 with no edits) showing file size of 30.73MB - both in Windows and LRC Metadata.
2. Exported to jpg with no changes, and at quality of 100%. File size reduced to 13.86MB.
3. Did “Save as" (name change only) from Windows Photo Viewer - resulting in no file size change.
4. Then from LRC, did export from jpg to new jpg with changed file name (closest process in LRC to a “Save as”). File size then increase from 13.86MB to 13.97MB according to LRC metadata (confirmed this file size in Windows).

PUZZLING: the second save in LRC on the jpg not only did not show additional compression (implying additional loss), it showed a larger file size - implying additional data. Again no edits were applied outside of file name change. Ideas?