@Littlek0406 I've just been through something very similar with my niece (G) who lives with me. A year ago we were notified by social services that explicit photos and videos of her had been found on the internet. G would have been 12 when they were found, and was probably 10/11 when she took them - utterly horrifying. She fed us a story about being coerced and threatened which didn't make a lot of sense (the facts kept changing and when we asked to see these threatening emails and facebook messages she claimed that they'd somehow disappeared as soon as they were read) but we didn't question it since we figured she was probably just too embarrassed to admit that she'd been manipulated into 'wanting' to take the photos. We took her phone, strictly controlled her internet access, bought special internet filtering software for teenagers (the one we use is called Circle if you're interested - there's a physical component you attach to your wifi router and control from an app on your phone). We assured G that she wasn't in trouble and that she had nothing to be ashamed of but that unfortunately we would all need to make more of an effort to keep her safe on the internet.
A few months after this, her school began in-person teaching again and she spent much less time on her computer. Her behaviour improved to the point that we felt comfortable buying her a very basic phone (no camera or internet, only calls and texts) for her last birthday. Then, right before Christmas, the police came to our house explaining that they'd had a call from a man several hundred miles away who hadn't been able to get hold of G for 2 days and was worried that something had happened to her. We confirmed that she was fine and that there was nobody from that city who had any business inquiring after G. We went through her phone and found some extremely graphic texts, which the police confirmed had been sent by the same number that had called up asking them to check on G. Later that day, I received a message on social media from an account with the same name that the police had given. He was in his 20s and had been talking to G for a year and a half. He introduced himself as her boyfriend. He claimed to believe that she was 20 and seemed genuinely disgusted when I told him that she would only have been 13 when they first made contact. The social worker set up a video call in which G claimed to have no knowledge of any of this, and concluded that someone must have been impersonating her online. After many tears and tantrums I managed to get G to admit that she'd been the one sending the messages, but she maintained that she'd been coerced somehow. Eventually she conceded that she hadn't been forced to talk to this man, and that she'd just made up the outlandish stories he'd heard about her for a laugh (this man thought she had a younger sister who'd killed herself, an abusive ex-boyfriend, dead family members, cancer, you name it - she was like a one-woman soap opera). Her laptop and phone are still in police custody. The school have loaned her a replacement laptop which she is not allowed to use at home unless I am sitting next to her to supervise every single click. If I look away from the screen, she has to close the laptop. If I'm not in the room, the laptop goes with me. It's exhausting but it keeps her out of trouble.
The other condition for her laptop use is that I have to know the password. This has allowed me to go into her browser and access the list of saved internet usernames and passwords. If you're able to do this on the devices you confiscated from your daughter I would highly recommend it. I was able to access all G's messages with the man who'd called the police, in which he repeatedly acknowledged that she was only 13, so his whole shocked catfishing victim routine was all a lie now that we’ve got proof we’re able to press charges against him. I also discovered that G has a secret instagram account, 8 different email addresses, accounts on a bunch of chatrooms I've never even heard of, discord accounts, quora...I'm barely halfway through and cannot believe the volume of filth I've found. G was exchanging photos with about 30 different men, all aged at least 25. I know it's horrible to imagine, but it's highly likely that what you know about is only the tip of the iceberg.
As for where all this comes from, I think it's a mix of loneliness and possible sexual trauma. I noticed that G's messages often mentioned being lonely and depressed, and wanting someone to be her friend. You mention that your daughter seems autistic so she possibly also struggles to fit in socially. At the time when these messages were sent, G was doing all her lessons from home in lockdown and was feeling very isolated from her peers. I got the impression that she just wanted companionship and validation but that the only way she could get it from strangers on the internet was by simulating sex acts with them. The sex talk was always led by them and I could see her picking up all the phrases and gross fetish talk from them as these relationships progressed. It’s been suggested to me that G may suffer from Borderline Personality Disorder (BPD), so maybe that’s worth looking into for your daughter too. It isn’t usually diagnosed in children under 18 but if CAMHS think she’s exhibiting BPD behaviour it may help inform their treatment of her.
I also suspect that G is acting out sexual trauma of some sort. I don't think I'll ever know exactly what because she is such a chronic bullshitter, but there's usually a grain of truth in whatever stories she tells me. I now keep an eye out for any clues of this nature in anything she tells me, and I think it’s worth thinking in a very dispassionate way about any and all adults (of either sex) who could have abused your daughter.
Sorry for rambling, it's the first time I've been able to talk about this with someone who's been through the same thing and I got a bit carried away. If you'd like to send me a message please do.