1) People who want to know the mechanics usually look up a guide on youtube by MTQcapture. Be aware that these guides go into so much detail about each mechanic that it makes them look more complicated than they actually are. The guides can spend a minute explaining a mechanic that only lasts a split second and is obvious in the moment and end up leaving you more nervous than you were before. Fights are designed to introduce mechanics to you one at a time and then test you by doing a few of them at once, if you don't figure it out and die to it, it doesn't matter that much.

2) You are referring to the limit break (usually abbreviated lb). The melee DPS usually uses it on a boss because this is a single-target attack. In a dungeon, ranged DPS can use it on packs of mobs because their one is AOE. In raids the DPS may decide not to use it in case the healer needs it, but this depends on the raid, some of them are not as difficult as they once were because of the item level it syncs to. There is one raid at level 60 and one trial at level 80 where tanks have to use their limit break, but this is usually early in the fight and the bar charges unnaturally fast.

3) You can /shout at Limsa Lominsa, click anyone in front of you and apply to their free company or look at https://na.finalfantasyxiv.com/lodes...munity_finder/

You can ask anyone with a crown icon to invite you to the Novice Network, where you can ask questions.