Need help understanding ranked games

So I am just wondering why/if the ranked system puts you in games with people close to your mmr? Or is that not the case? Does it put you in games (ranked games) based only on your actual rank? Someone told me it was based on mmr regardless of rank. For instance, if a high ranked player goes to play his promotion can he expect to be put up against any other mmr? Or can he expect to be put up against players close to his own mmr? Doesn't that defeat the whole purpose of a rank? Isn't your mmr score your true rank? Also, why dont they have a solo only queue? For those of us who play lol by ourselves its kind of unfair to have to fight against teams of friends. For instance, Rocked League has a solo Queue where you can not have parties.