Hey @Tony_Siciliano…I think both of your questions are similar and related.
First of all and as you know, many photographers choose to apply sharpening to their initial workflow while processing an image at its native size. I hear this called “RAW” sharpening, or “input” sharpening. I’m sure you’re familiar with this.
Once an image is going to be considered for print, many photographers will upsize the image to the appropriate dimensions for print, then apply “output” sharpening after upsizing the image. Too much sharpening prior to upsizing can exaggerate artifacts, so some photographers do the majority of their sharpening once the image is upsized and ready for print. Again, it depends on how picky you’re gonna be about the final product.
I’m really not sure the degree to which labs apply sharpening and again, it would be best to have the lab answer your question. My instinct is that some people will send a file to a lab without upsizing, and ask for a print enlargement. In this case, they may take the liberty of applying sharpening. This can be done a variety of ways, but it is interesting that some of the upsizing algorithms in Ps actually do apply some sharpening as well. Others don’t (I’m no expert on this!).
So, I don’t know if I’ve answered your question. Again, I’m no expert, but have an interest in the process.