My uncle has lived in LA for a long time. I got to visit him after graduating high school and get a grand tour of the city. He's in a suburb of LA proper but everything Kitty Pryde said was part of my experience there. It's a huge city broken up into little districts that have their own distinct feel. We went to downtown central LA which is a big manufacturing and garment district. It's run-down and a place where they shoot post-apocalyptic movies thanks to it being the real deal. We went West out to Venice Beach and it was just as crazy as seen in a million movies that feature a "california beach" scene. Folks were rollerskating by in the perpetually sunny weather and weight lifters worked out right on the beach. We toured up Rodeo Dr and took in a play right there in a little hole-in-the-wall theater. Dinner for the night was at an Indian place down the road with some of the cast. I remember all the tiny shops there - the store fronts were pretty miniscule. We went down into another sector of LA and got to dodge bullets and crackheads as we went and got the best BBQ I've ever had at Dr. Hoggly Woggly's Tyler Texas BBQ. Okay, that's a minor exaggeration - the notoriously scuzzy house across the road wasn't serving any crackheads at the time we were there but it's still a spooky district to be in at night. Grocery shopping was mindblowing for a kid from up North. I was one of a handful of white folks in a store that was blaring Russian, Spanish and possibly Japanese up on the speakers for in-store announcements. English was an afterthought.
Pretty quickly you get used to seeing people of all different skin tones all over the place, you learn by word of mouth what districts are dangerous - although much of this is folks being sensationalist. Like any big city there are loads of neighborhoods with problems and there are really scary gangs in LA but if you're a tourist you're more likely to get politely redirected back to the freeway so the locals can continue their blood feuds than to have anything bad happen.
Smog and horns and cars cars cars. Everyone drives everywhere because of the sprawl. When the weather is good, you can see the hills surrounding the city. When the weather is bad, an inversion layer sets in and smog gets trapped and it's a hellish yellow-brown tint in the air that gets worse depending on where you are in town. Bleargh. Just gross.
Hollywood is its own bizarro world nestled in this humongous city. It's supremely weird because the entertainment industry is the biggest employer around, but there's always layoffs and a tiny tiny number of jobs for a humongous population of hopefuls so you'll see long lines of people queued up for auditions or people milling around carrying equipment because they're on the job right now shooting a movie. All of this is outside of the studios themselves. The closer to that area you go the more likely you'll see someone you've seen on the big screen. The correct way to approach a celebrity is to remark, "Wow, you're a lot shorter in person!" This establishes the right dynamic at the start.
There's a lot of people living in the city going about the business of raising families, going to work and living life. That's no different from any city on Earth. What everyone I've known has said about LA is that it's a horrible place because of all the phony people that live there. They say this as they book their tickets to head back there because it's home and are honestly surprised if you asked them when they were moving. Loving to hate LA is a special prerogative for people that live there. I think it's similar to how New Yorkers fiercely defend their shitty, overpopulated nightmare of a city. Just typing that has already triggered the spidey sense of a bunch of New York apologists who are getting ready to bull in here and smack me down for being some kinda jerk. LA native would put on their sunglasses, smirk at the uncultured idiot from some backwater nowheresville and drive off to their next audition.
Specific to your story, you have a young lady who was a child actress and just hit 18 in Hollywood? Hahahahah oh my. Are you sure you're not prepared to write a cautionary tale or erotic fiction? The entertainment industry can be horrible to women and with the multi-billion dollar adult industry filming right there in the same Hollywood hills apartments as are used for interior shots you have a lot of avenues for a young girl to be pressured and taken advantage of my master manipulators. Hell, even low level scumbags are in profusion because there's a certain desperation and story about los angeles that pervades american culture: if you go to Hollywood, be prepared to give up your innocence for a shot at being the next big thing. Of course this is the story and mythology that overshadows everything. The reality is a lot sadder and grimier. Come to LA with your acting resume, get a crummy day job and a postage stamp of an apartment and then hit every audition you can. You're more likely to spend time on your feet waiting than on your back with some "casting director". Get rejected a ton, maybe pick up some bit work as an extra. Since your MC has acted before they probably already have their Screen Actors Guild membership. If they don't they will have to go through them and get accepted before they can get any roles - the town is fiercely unionized.
That's a lot of words about LA. I was fortunate enough to visit it a few times and each time was pretty surreal. I got to videotape a wedding between a film star and his nightclub entertainer wife at the Church of Scientology's hotel/megacenter. I got to run camera for a man-on-the-street interview hoax. I auditioned for a voiceover part because the original person didn't show to their interview and got to feel like a complete minority at a grocery store. I went to the beach, went to some parties, and had a great time. I wouldn't want to live in LA but it's a great place to visit every so often.