and how would an elder start looking for friends or recreational activities for seniors? I don’t even know what’s available.
Not sure how regional this is, but a very midwestern American thing is the "old man coffee club". This is not an official name or organized sanctioned event, but its such a regular occurrence that everyone knows what it is. Old men will meet at a diner or restaurant where they will drink coffee and chit chat. Its usually a weekly meet, but in some localities its a daily thing.
I don't precisely know how to tell you to find your local one. I know mine is at a local McDonalds on Wednesday mornings only from happening to be there at that time and immediately recognizing it for what it was. See if you could find that for your dad. I look forward to my retirement days attending my local old man coffee club (and to be complaining about the young presumably).
As for formal events, look into your local "Center on Aging" or "Senior center". This is usually a public organization that hold events for seniors and many times has a regular lunch hall (very reasonably priced!).