Not much to add to Barry's excellent post above. I've been on HT (Zoladex) since Jan 2020. Subcutaneous implants every 12 weeks at my GP surgery. Supposed to come off Jan next year after 3 years treatment. For me personally the biggest effects have been lack of strength, general tiredness, the occasional hot flush, and weight gain. Nothing drastic, all annoying but manageable. In a way knowing the cause of it and that there's an end in sight down the road helps.
I had 20 fractions of EBRT two years ago. The daily visits were a grind but I soon got into the routine and after that it all went very quickly. Some details will vary depending on the policy adopted by the radiology dept so your Dad's experience may differ from mine. In my case I was required to have just the right amount of fluid in my bladder for each session, and an empty rectum. For the bladder, they will advise your Dad of what they expect (some places want a completely empty bladder) and give him instructions for the routine to follow to achieve it - how much water to drink and when, when to go to the toilet before each appt etc. I think empty rectum is pretty much universal. It has to be as small as possible during treatment to avoid damage from the radiation beam. I was given a ton of laxatives and had to administer a micro-enema (a small plastic tube of liquid provided by the hospital) an hour before each session. It's not something anyone would choose as a hobby! That said, once in the routine,,,etc. They do an ultrasound scan before each EBRT session to check everything is in the desired state.
Regarding side effects from the EBRT. I've had some bowel trouble ever since, attributed to radiation damage, but it's gradually getting better. I also had a nasty bout of radiation cystitis last year but got it sorted and haven't had any trouble since. Bowel and bladder are basically the things that are usually effected. Please note that that these were my personal experiences only, that I give as an example. A great many men suffer no side effects whatsoever, and your Dad may well be one of them. In any case any effects he does happen to suffer are very, very unlikely to be life affecting but will most likely be like mine - just a bit annoying now and again! Best wishes to him and don't worry he will be in the hands of true professionals and they will take the best care of him.