The only explanation we get in The Rise of Skywalker film about Snoke’s backstory is that he was being controlled by Palpatine the whole time. He tells Kylo Ren that he’s been every voice he’s ever heard in his head, and that includes Snoke. We also see a baby version of Snoke in a giant test tube so apparently he was created by the Sith Eternalists too.
As far as the rapid pace of the story of The Rise of Skywalker goes it works. Then fans start thinking about Snoke after watching the film and a more detailed explanation is wanted. The novelization gives us that. It says that Snoke was created as a test by Palpatine to see if Kylo Ren was a worthy heir to his throne.
The Emperor was pleased that Ren was able to destroy his creation and by killing Snoke it further cemented his pathway to the Dark Side. He also now believes Kylo Ren is as powerful as Vader and a fine successor to carry on the legacy of the Sith.
This information would’ve been helpful to hear in the film. This movie is the culmination of the Sequel Trilogy and finding out why the main villain through the first two films’ ultimate purpose was would have tied up a ton of loose ends.