Welcome to the site! I'll try to answer your questions.
OK, you have a little misunderstanding about the nature of Stack Exchange sites. Stack Overflow was the first and original site, and then Stack Exchange was setup to create further sites along different topics. These days there are SE sites for everything from coding to parenting via philosophy and bicycling. You can see a full list here.
We don't have groups, as such, but each sub-site (usually called sitename.stackexchange.com e.g. apple.stackexchange.com but sometimes with their own redirecting URL such as askdifferent.com for our site etc) is essentially a wholly seperate entity, with their own topics and their own rules for what is on-topic etc. Clearly asking about nappies on a Ubuntu site is not appropriate! ;)
What can link them, is you - your user account can be linked to many such sites, you have a profile on each site, and nominate one (usually your first) to be your parent profile under which all others are linked.
When you do this, if your reputation score on one SE site is sufficient, you can a 100 association bonus, the purpose of this is to remove the early privilege limitations whereby you need to earn reputation in order to do more than the basics - as the message says when you are awarded the bonus, you are trusted to know how to use an SE site, and don't need to prove that again. The reputation is added to the profile in the site you just joined, purely to remove the new user restrictions - you can't add it to your rep from another site. Overall, it is calculated in your total rep across all sites, but you cannot, for example, join 50 sites and get a 5000 bonus on your primary profile...
Now, as for Objective-C programming, the main coding site is still Stack Overflow. There is an Apple group, as you have found, but programming in anything other than user facing utilities like Automator, AppleScript, and a bit of Terminal is off topic here. Likewise, a similar restriction applies to the Android site.
The Stack Overflow site is the primary source for all programming questions, regardless of your platform or language of choice. You can filter by tags to ensure you only see questions which are appropriately tagged with Objective-C or Xcode etc, but they are still all included in the same site.