It's a magnificent movie, if you have the references.

Rating: 9/10 (30 September 2019)

The movie can be divided in 3 parts. The downfall of Rick Dalton's career as an actor, his journey in Italy and his return in LA. In those three parts Cliff Booth stays with him. On the other side we have Sharon Tate, who is in LA in February stays there and gets pregnant, in August, when the two men return with Rick's new wife she is in LA without her husband and with Jay and two other friends.
The movie's title is very well chosen. Fictional stories begin with once upon a time... , they take on the story of characters, their downfalls, their problems. "In Hollywood" can be translated into Tarantino's love to Hollywood in the 60's; it is a reason why there are so many references to classic movies, actors directors etc.
If you waited for the characters to meet, you waited a long time, it is only in the last scene when Sharon Tate talks to Rick. The movie is basically showing you this: Rick's career, his friend/stuntman who's supporting him and Sharon Tate's 6 months living in Hollywood. If you're familiar with the Manson murder and Tate's death you'd expect that the 4 hippies would go to Polanski's house and kill her like it really happened. There was a twist in the movie where the hippies didn't kill anybody and attacked Rick, Cliff and Francesca instead. So it's not like reality, it's a fiction( Once upon a time...).
I've read other reviews where they complain that nothing happens, there is no drama and that the movie is just a love letter to Hollywood. it happens, only, it's not between the three characters, here's an explanation: Rick Dalton is struggling with his carrier he has difficulties of his own, those difficulties aren't related to Cliff or Sharon, it could be called individual drama. On the other hand Cliff is having his adventure with the hippies, again not related in any way with the other two characters. And then there's Sharon Tate who's having a perfect Hollywood life, you'd think that she's there because she'll get killed in the end. No such things happens. Sharon Tate, Jay and Roman Polanski are just a reference to the Manson murder.
To understand the movie, one must know the context. First: The western years, actors having a hard career, caught in business tricks. Then the hippie movements in the USA and the Vietnam war and last the Manson murder. Each character has its drama in one of those contexts: Rick with being the one with the hard career downfall, Cliff meeting the group of hippies in Spahn's Ranch and Sharon Living on Cielo st, in her case nothing happened, there was a twist in the story.
So, in conclusion the three characters are allegories of what Hollywood was in the late 60s. you shouldn't have expected any drama between the three characters. It's a magnificent movie if you have the references.