They are addictive. I think you need to be careful with it. We have to limit DS1 very very carefully or he will be on it literally 24/7. For us that means time limits including electronic guards against it (which are more a reminder than an absolute wall), harsh penalties if he's caught trying to sneak around them, and we try and keep on top of it verbally as well and model healthy usage ourselves (we don't always manage).
I know it's a lot of effort, but I think it's worth vetting each and every thing they do on there - particularly if you have a child who's susceptible to it.
Plenty of games and websites and apps and even TV programmes are specifically engineered to keep delivering little "hits" of dopamine - one of the chemicals that makes us feel happy and motivated. The reason they do this is eyeballs on ads - the longer they keep you looking at it, and the more they keep you coming back, the more money they make. The younger the child/teen, the more susceptible they are to this. Things like ADHD, anxiety and ASD can also make a person more susceptible, as well as more temporary issues like boredom, overstimulation, stress, feeling left out/ignored/unhappy, lack of attention etc. And bear in mind they develop this stuff to be aimed at an adult (with a mature system for dealing with distraction), neurotypical, mentally healthy and so on - so a child already being young is at a disadvantage and then having any other of those factors, even more so. You wouldn't let them use alcohol, nicotine etc - even though you don't have a problem with your adult friends doing these things. Although it's more tricky than that, because tech is so ubiquitous and they do need to learn to navigate it somewhat. Being aware that it's an issue, and how and why is the first important thing to get your head around.
I do think if you have a child who seems especially pulled in by it, have a look at what they're doing. If it's youtube - which channels? Are they waiting for the latest episode by this or that person, or are they just clicking randomly on the next thing? Are they watching something of substance? I don't necessarily mean it has to be worthy or educational. Everyone will also have their own views on things like content, language etc. But there is stuff on youtube which is the equivalent of clickbait - literally, buzzfeed/bored panda lists fed into a computer, read out by a robot voice and displaying the accompanying picture and those kinds of channels churn out hundreds of videos per day. If they're watching that kind of stuff, they're just being used as ad fodder and they may as well do something better with their time. Also, I'd recommend changing settings on youtube, netflix and so on to cancel autoplay - so they actively have to decide to click on the next thing.
Games - there are games which are interesting and challenging, something you can play with a friend or just plain mindless fun. But there are also games which are set up so that nothing really happens and you're not really doing anything, but you're constantly either levelling up or teetering on the edge of some kind of paid content - again - clickbait in game form. If the game is sending constant notifications, that's usually a red flag. You can block notifications, which I'd recommend doing for most apps anyway, but this might not solve the problem. Have a look at how the game mechanics work and see if there's a good way of timing sessions on it. For example, you can say I'll play three matches of FIFA. But some games are set up so that whichever point you try to put it down, it feels like you're just about to get something so you think oh I'll just finish this one thing - and then something else is about to complete, so I'll just wait for that - etc and it keeps you on there, eyeballs on ads.
Nine is probably too young for social media, but the same kinds of things apply - with several other issues which have been covered many times by others so I'll skip that here, but the same kind of conversations can be had, and just to keep the dialogue open so that it's normal. Otherwise an invisible wall is drawn where they feel like apps, social media, internet stuff is "their world" and it's too huge and complicated to even start explaining it to you and you're "old" and "out of touch" - I do reckon if you start the conversation at this kind of age there's more hope of keeping up.
And also just talk to them about the dangers of this kind of thing. Talk about how the games, apps, media they use make them feel. If they feel it's a fun thing that they can pick up or put down, fine. If they are constantly in thrall to something thinking when can I play X again/when will my mana/coins/time have refilled, that might be a sign that it's taking advantage of them. If they get the feeling after watching a youtube video "I can't wait for the next episode" vs just wanting/needing to mindlessly click onto the next thing to prolong the sort of hypnosis feeling - you see what I mean? It might also be useful to get them to categorise media into different types. Something they want to use and allocate time towards (also, even steer them into more productive uses of tech - such as digital art, digital photography and editing, film or music editing, stop-motion animation, coding/programming, language learning, courses on Khan academy, educational documentaries, tools they can use to learn or connect with people), vs something they use to kill time e.g. when they have to wait for a long time and are bored. I'd only ever do the latter when I actually have "dead time" e.g. long train journey, hospital waiting room, etc. (Which, to be fair, does a 9yo often have? Not really). The other use of the "time killer" apps for me is when I don't want to think about something stressful - again, worth thinking if you want a 9yo using this as that kind of coping strategy or whether you can help them develop better ones.
Separating tech activities out helps with having restricted screen time, and knowing that their use of time-killing type apps are eating into their time to do actual fun/useful stuff on the screen, but you might have to help them to ringfence their screen time to achieve this, since their self-control probably isn't developed enough for them to do it by themselves.