Where’s the multiplayer in Clicker Heroes mobile

Currently, they do nothing. In the future they will be used to upgrade relics. You get them by salvaging relics.

http://clickerheroes.wikia.com/wiki/Relics

What do Forge Cores do?

As of v0.19, Forge Cores does absolutely nothing. Fragsworth (ClickerHeroes dev) has vaguely mentioned that "forge cores will be used to upgrade items". By items, it's safe to assume he means relics. As far as the scope of what Forge Cores will be capable of, only time will tell when it's implemented. Right now it's just all speculations. How do I get Forge Cores?

You can obtain Forge Cores when you salvage a relic. The amount of Forge Cores a relic is worth depends on the rarity as well as the level of it. However, the value of a relic seems to leans towards the level of it more, rather than the actual rarity. As an example, a level 50 common relic will give more forge cores than a level 40 rare relic.

Let's take a look at the source code, shall we?

The script responsible for (currently) the fish and the bee is heroclickerlib.managers.HiddenObjectManager.

Inside that script we find a function private function onHiderClick(param1:MouseEvent) : void.
This function is responsible for the on-click event when you click the fish.

private function onHiderClick(param1:MouseEvent) : void
  {
     var _loc4_:* = 0;
     var _loc5_:* = NaN;
     var _loc6_:* = NaN;
     this.removeHider();
     if(Rnd.boolean(0.44))
     {
        _loc4_ = 1;
        if(Rnd.boolean(0.04 + this._userData.ancients.doubleRubyPercent / 100))
        {
           _loc4_ = 2;
        }
        this._userData.addRubies(_loc4_,"hidden_object",-1);
        _loc5_ = this._battleManager.display.mouseX;
        _loc6_ = this._battleManager.display.mouseY;
        this._battleManager.uiManager.battleUI.transientEffects.showGeneralMessage(_loc5_,_loc6_,_("+%s Ruby!",_loc4_));
     }
     var _loc2_:Monster = new Monster();
     _loc2_.id = 2;
     _loc2_.level = this._userData.highestFinishedZone + 1;
     var _loc3_:Number = this._userData.getMonsterReward(_loc2_) * 10;
     _loc2_.isBoss = true;
     this._battleManager.itemDropManager.goldSplash(_loc3_,_loc2_,ServerTimeKeeper.instance.timestamp);
     HeroClickerSoundManager.instance.playSoundEffectById(36);
     this._battleManager.uiManager.battleUI.transientEffects.showGeneralMessage(580,152,"Yay!!");
  }

Two function calls are important here:

1. if(Rnd.boolean(0.44)): this internally checks if a (uniformly distributed) random number is less than 0.44. This evaluates to a chance of nearly exact 44% of at least one ruby.
2. if(Rnd.boolean(0.04 + this._userData.ancients.doubleRubyPercent / 100)): This rather lengthy call checks if a random number is less than 0.04 + the additional double ruby percentage of your ancients. This means that in 4% of all cases of getting a ruby, you get 2.
   Without any ancients, the chance of getting 2 rubies is about 0.44*0.04=0.0176 => 1.76%

The average amount of rubies you get after 1000 times clicking on the fish is: 444,4 => 44,44%

440 times at least one ruby
=>  440*4%  =     11 times 2 rubies = 22    +
=>  440*96% =  422,4 times 1 ruby   = 422,4

==> 444,4 rubies => ~444,4/1000 = 44,44% chance of getting one ruby on average

The expected amount of rubies per click is then 0,4444.

I used JPEXS Free Flash Decompiler to get to the source code.