We have heard the Spanish value family time on holidays, closing most restaurants, bars, entertainment venues and monuments. We will train Barcelona to Seville on New Years Day.
On the 5th we’ll trail from a Granada to Cordoba a for two nights stay. What should we anticipate and be prepared for regarding realities of limited food options on those two holidays? What about access to monuments and night life?
If by "monuments" you mean indoor attractions, you should be able to get information about holiday closures on the individual websites. Information about early closings may not always be posted on the Internet, in my experience. If you stop by the tourist office in each city, you may be able to pick up a summary sheet detailing the holidays hours at all the key sights. I found such information generally reliable except that Granada wasn't prepared for the imminent change to summer hours.
In the old town area Seville where we stayed at on the New Year's day eve 2019 after daytrip to Cadiz many if not all the restaurants were open. The Flamenco clubs were open. We were in Malaga on 3 Kings Day; there was a big parade that we watched as they were throwing candy to the crowd; the Gibralfaro was open that day