This is a really interesting question, that I don't have a great answer to. However, I may be able to provide some historical context on the practice of concubinage in Korea.

In the Koryo Dynasty, the king was allowed to have multiple wives, but concubines were explicitly forbidden to everyone else. However, the practice of concubinage was officially established in the Joseon Dynasty, period and persisted in Korea up through the Japanese occupation. The Joseon Dynasty set up a monogamous system whereby there was one husband and one wife, but numerous concubines were allowed and acknowledged, making it different than polygamy or having a secret mistress. During these periods, the wife of a man who could afford a concubine but didn't have one, could actually be looked down upon by the local society and judged harshly. So, especially in the Yangban class (aristocratic), a wife was expected to find a suitable concubine for her husband.

Concubinage was officially abolished in South Korea in 1948 with the enactment of the Constitution of the Republic of Korea. However, even after that the practice continued unofficially both due to the shortage of men as a result of the Korean War, and lingering conventions despite the new law. Additionally, beginning in the 1960s, well after after the establishment of the concubinage system, it became common for men to take up a concubine prior to divorcing their wife, then remarrying with their concubine.

There are elderly women still alive today who were concubines in the past, as it seems your grandmother was. It is difficult for historians to find them as most don't like talk about it, but there are numerous examples of people living in rural areas where secrets are hard to keep, and everyone knows who was the wife was and who the concubine. Among these people, every now and then there were examples of the wife and the concubine living together after the husband dies. There was a case earlier this year where a 70 year-old former concubine murdered the 80 year-old widow, after they had lived together for over a decade following the passing of the husband.

There have also been a number of cases where people knew their grandmother and great grandmother as a certain person, but when looking through historical records, like the Family Census Register or a family tree, would find that someone else was listed as their relative. This was because typically a concubine's children would be recorded as the wife's children. There were also some limited cases where the concubine was the sibling of the first wife.

Unfortunately, I wasn't able to find a good source for the day-to-day life of a concubine, but I'd be surprised if there weren't some sources in Korean.

Thank you so much for the information! My mother lived a hard life as her mother was only trying to provide a male heir so she was never really valued and often left to fend for herself. Like you had mentioned, her mother is officially listed as the wife of her father but she was never recognized by her father as his child. My grandmother and my mother lived on my grandfather’s land but that was it. Before you provided some insight this was all I knew.