Explaining a virtual world where dangers exist to a 6 year old will require a lot of comparisons to dangers in the real world. A critical part of shielding her from those dangers is limiting and monitoring her use of the device. So the easiest explanation might simply be that for her safety, you must use the phone with her, and you don’t have the time to do that yet.
That doesn’t get you off the hook completely, though, as you still need to discuss dangers with her, and she might feel unimportant that you don’t have time to do something she would find enjoyable.
Strangers on the internet: If she understands what strangers are in real life (most young kids don’t), then get on the internet with her. Go to any social site with a lot of users and log on. Tell her roughly how many visitors/users that site has per day. Then you might ask her if she thinks you’re friends with all of those users. Listening to her answers may help you assess her understanding of the internet and help you to teach her about strangers on the internet. When that’s settled, look at some profiles. Ask her if she would want a stranger to know that about her. Again, the concept of stranger here is critical to understand, so her answers will be a guide for you. If she’s unconcerned, that’s a whole new conversation, an extension of that about strangers in real life.
Bullying is something most kids understand. You might want her to know that bullying can also occur on the internet on social media platforms and has caused a lot of sadness in many kids. That might be better saved for a few years, though.
Addictive behavior may be easier to explain. Pleasure is something we all want to feel. Ice cream is great, but what would happen if you ate ice cream several times a day? Again, her answers can be a guide for you.
Addiction is compulsive behavior that interferes with our lives, or more simply something we find pleasant when we do it but that is hard to stop. If she gets used to this, getting to eat ice cream several times a day, even if she’s getting, say, cavities, or can’t fit into her favorite clothes, would she really stop if the ice cream was still there? Odds are, no. Explain that she might always be thinking about it, and those thoughts would interfere with her finding pleasure in other things. Tell her that even adults can’t stop; that’s why they smoke when it’s bad for them. When they try to stop, all they can think about is how much they want “one more” cigarette. That’s addiction. If she were on the internet all the time, what other things would she miss out on? Why are those other things important?
You limited your question to those specific issues, but another issue I’d be concerned about is manufacturers’ use of attractive content to prey on the desires of young people for goods or services. But that might be a question for another day.
Edited to answer question in comments.