Beth Sanchez was the original daughter of Rick Sanchez from Dimension C-137. She lived a childhood similar to other versions of Beth, where she had Rick make inventions for her such as Froopyland. But in unknown circumstances, this Beth died before reaching adulthood, by the time Rick was 35, meaning she never had Morty and Summer.
In a "totally fabricated" memory projection in "The Rickshank Rickdemption", she and her mother Diane Sanchez are both killed by a bomb sent by another Rick. At the time, it was unknown if Beth truly died along with her mother.
Through Birdperson's memories in "Rickternal Friendshine of the Spotless Mort", it is revealed that he and Rick once fought a group of Ricks with sci-fi haircuts who said, "Killing us won't bring her back", possibly referring to Beth. This episode also confirmed that Rick's original Beth had died, and that at some point after Rick was 35, he started moving to dimensions where his daughter had survived to adulthood. There are many other dimensions where Beth died as a child, from which originate all the other Ricks seen in the series who have similar backstories to Rick C-137. Memory Rick accused Rick of being one of the "creeps who moves in with abandoned adult Beths", but Rick insisted it was more complicated than that.
In "Rickmurai Jack", it was confirmed that Beth had died along with Diane in the same way as shown in "The Rickshank Redemption", and Rick had tried to track down the Rick who killed them. Rick never was able to find out, and eventually he helped build the citadel. He traveled to a new universe where Beth was alive and abandoned by him, and moved in.
