SpriteKit, iOS and Swift questions are all fine on Stack Overflow. However, it is worth ensuring that you've shown what you've tried, you're sticking to one question per post, and that your question is not too broad. In the latter category, questions about design or architecture are considered (by the community) to be either too wide or subjective. The aim is to have questions that are definitively answerable, rather than topics that could branch out into discussion.
I can see the Stack Overflow point here - if anything and everything was on-topic then it would become impossible to moderate. If you have a particular thing you want to ask (I don't use any of these technologies) then by all means outline it in this thread, and I'll see if it could be bashed into shape as an "on-topic" Stack Overflow question.
Yes, unfortunately this is too broad for Stack Overflow - there's lots of different ways to do this. I'd add, what is Agar? I understand it's a game, but do people need to download it to understand your problem?
Do you mean a leaderboard refreshed from the internet, involving all registered players? If so, I imagine your app would read some data on a schedule whilst the app is running, and cache the data locally. Perhaps it could refresh the data every few hours? I appreciate you want the data live, but you also don't want to munch through people's data allowances too quickly!
On the remote side, you'll need to offer a web service containing the data. This can be a simple JSON document with the necessary data, which you can decode on the mobile device. You can use whatever technology on the server side you like - PHP, Ruby, Python, Node, .net.