Star Trek: Picard features the return of Whoopi Goldberg as Guinan - but what has she been doing since Star Trek: The Next Generation? El-Aurians may look like human beings, but in reality, their physiology is very different; they age at a much slower rate than humans, and as a result, their lifespan covers many centuries. Guinan, one of Jean-Luc Picard's closest friends, first visited Earth in the 19th century - and remained involved with human affairs throughout the rise of the Federation.
Guinan probably isn't immortal, but she was probably around 570 years old when she went to work on the Enterprise-D. She soon proved herself an invaluable member of the Enterprise's crew, becoming a trusted adviser to Captain Jean-Luc Picard and others. Star Trek: The Next Generation avoided revealing many of Guinan's secrets - but Picard learned some of them when the series had come to an end, in Star Trek: Generations.
In Star Trek: Generations, Picard learned Guinan had been one of the passengers aboard an El-Aurian spacecraft in 2293. Her ship encountered a time-space phenomenon called the Nexus, a doorway to a dimension she had tried hard to forget. Guinan's information proved crucial when Doctor Tolian Soran, another El-Aurian who had been exposed to the Nexus, attempted to return to the Nexus. Without Guinan's guidance, Picard would never have come to understand the Nexus or Soran's purpose. Star Trek: Generations became a turning point in Picard's life, with the Enterprise destroyed - and Guinan does not seem to have decided to serve on the next model.
Guinan was among the guests invited to the wedding of Deanna Troi and Will Riker in Star Trek: Nemesis, where she traded pleasantries with Geordi LaForge; when he asked if she'd ever considered getting married again, she told him twenty-three was her limit. The scene was expanded upon in J.M. Dillard's novelization, with some of the dialogue switched up to suggest Guinan didn't see the point of marriage; "Why buy the Denubian seacow when you can get the milk for free?," she quipped, to Georgi's amusement. Neither the film nor the novelization offered any hint as to Guinan's activities at the time.
Star Trek: Picard has moved further into Star Trek's future, to the end of the 24th century. Jean-Luc Picard had withdrawn from the galaxy, presumably including from his friendship with Guinan; the first season saw him begin to boldly go once again, although he didn't reconnect with Guinan. Star Trek: Picard season 2 features the return of Guinan, and hopefully with finally explain what she's been doing since she left the Enterprise.