There may be some slight variations in protocol between RT treatment centres, but this is what mine does...
They will target whole prostate and base of seminal vesicles, or they risk missing some cancer that hasn't been spotted. Generally speaking, you can't go back and have more treatment for anything that's been missed, as you will have had max lifetime dose in the area during your first treatment. (There are some exceptions.)
If you are a high risk patient (PSA >= 20 at diagnosis, or T3x, or gleason >= 8), they will target all of the seminal vesicles.
I don't know the scope for including individual lymph nodes, or even if that's advisable, verses hitting them all. Again, if one is missed that had micro-mets (mets that are too small to show up on a scan), the scope to go back and retreat might not be there.
Sometimes just the prostate is treated. "Prostate and nodes" is also a common treatment pattern which includes the main pelvic lymph nodes (sometimes call "whole pelvis" by oncologists, although that's something completely different to radiologists for treating pelvis bone mets which is not used on curative treatments).
It's important to understand that RT does most damage to cancer cells. It damages the DNA of all cells, but cells all have a DNA repair mechanism, and mostly repair themselves before the next day's treatment. However, cancer cells have a broken DNA repair mechanism (that's how they became cancer cells). They tend not to finish repairing DNA before next day's dose, so the damage accumulates in the cancer cells, eventually stopping them multiplying, if not killing them outright. So although other body cells get hit, they should sustain much less damage than cancer cells.
There are a number of organs at risk (OaR) that also receive a dose. These are the ones in contact with the prostate, and include bladder (and internal sphincter), pelvic floor (and external sphincter), rectum, urethra, various nerves and blood vessels. Normally, none of these have cancer cells, so they should recover from the RT, but there are cases where that doesn't happen as you know. There is an option (mostly only privately available at the moment) to inject a spacer between prostate and rectum to protect the rectum (SpaceOAR). It's not recommended for high risk patients (except on a trial currently running to discover how safe that is).
0.1 PSA is nothing to do with size (although it will likely have shrunk prostate). It shows how actively cancer (and to some extent, non-cancer) prostate cells are growing. Getting you pre-RT PSA down to < 0.1 is excellent, and improves RT outcomes.