My answer will be about China, and particularly, Tang Dynasty China (618-907 CE).

In Ancient China, not all concubines were equals. There was a strict hierarchy in the harem. The ranks themselves and their meanings changed depending on the Emperor, but what you generally had was the Empress at the top, about 4 consorts below her, then a certain number of imperial concubines (5 to 9), and below those an unspecified number of lower-ranked concubines.

The Empress was, of course, not a concubine. (Although it has happened in history that a concubine rose to that rank, Wu Zetian being the prime example. Other Empresses, like Empress Cao of the Song Dynasty, married directly into the role). Her marriage to the Emperor was the one that was officially recognized, and her children had the most legitimacy.

The consorts were the higher ranked concubines, and had generally not started their lives in the palace at that rank. They were the Emperor's favourites*. They were also usually the ones that had given birth to the older princes (that weren't the Empress's children). Since their sons had a possibility of acceding the throne, their mothers were treated with great respect. The line of succession in Ancient China was more complicated than simply older boy first—Emperor Gaozong, for example, was the 9th son of Emperor Taizong. Priority was usually given to the Empress's children, but not all Empresses bore sons. (During the Qing Dynasty, Noble Consort Yi [later Cixi] was able to become Empress Dowager after the death of the Xianfeng Emperor because she was the mother of his only son.)

*And the most powerful. While all concubines were women that came from good families, the consorts were also often the ones that came from the most powerful clans of the Empire, and also often had family members working beside the Emperor as ministers or generals.

The imperial concubines were ladies that had attracted the favours of the Emperor enough to be given a title. Some of them were also amongst the Emperor's favourites, others might have been in his presence a couple of times in their entire lives. Concubines that bore children were usually promoted, particularly when those children were boys.

At the bottom of the pyramid there were the lower-ranked concubines, many of whom might have never even met the Emperor. They lived their lives waiting to be called upon or to have an occasion to shine.

I'm going to assume that your question is more directed toward these women, whose names have been mostly forgotten by history books, since the ones at the top of the hierarchy had a role to play past their fertile age: being mothers. These lower-ranked concubines could retire as nuns in a monastery or work in the Palace (still hidden from view). The convent was also expected from the childless widows of an Emperor after his death, although some also chose (or were forced) to be buried with him. Wu Zetian, who I mentioned previously, was sent to such a convent after the death of her first husband, Emperor Taizong. Being a concubine meant living a life behind closed walls, with little to no occasion to leave the harem. The only people they would meet were the Emperor himself, eunuchs and the other concubines. Consorts were given many more privileges, but few of these women even came close to that rank.

TLDR: They became Buddhist nuns, worked in the palace, or sometimes died with the Emperor.