You need to own the latest version of Warcraft 3. Nadrex runs only on it and does not work on previous patches. 
Download link: https://eu.shop.battle.net/en-gb/product/warcraft-3-reforged?p=44414&pfm=warcraft-rts-family-page-remastered-games&returnFamilyUrl=https:%2F%2Feu.shop.battle.net%2Ffamily%2Fwarcraft-rts%3Fpfm%3Dfrom-checkout

AoS refers to Aeon of Strife - a popular StarCraft 1 map that gave birth to the entire genre, also known as Moba (multiplayer online battle arena). Players control a single hero used to fight for their allied faction, kills enemy creeps, heroes and structures which allows them to gain resources and experience to purchase powerful gear and unlock new abilities. The purpose of the game is to destroy the enemy Core Structure (also known as the Ancient / Nexus).

The mechanic known as last-hitting refers to the attack / spell that kills an enemy and rewards the killer with bounty (gold or crystals in Nadrex). 

There are 3 primary merchants. 

• The Main Shop - holds the list of all items, starting from basic to extremely advanced ones. Simply click on its pages to navigate and purchase items for your hero. 
• The Quick Buy (also known as Recommended Items) - provides 12 of the mostly used items that can work on any hero and in most situations. This shop provides the items with a single click as long as the player has the required resources.
• The Buffer Tome - this unique merchant sells buffs for your hero which are temporary (until the hero dies or the buff is dispelled). It also allows the hero to buy permanent points for one of the 3 attributes. 

Yes you can. The items are stored inside the Circle of Power of each player and once the hero of that player enters the base, they are instantly dropped on the circle.

Each recipe item contains the exact description of the components and the name of the shops where you can get them. Alternatively, you can simply one-click buy the entire item if you have the required resources, all you have to do is click on the recipe item on your inventory.

They are harvested every 90 seconds by the wisps located in the hero bases, but you can get them faster by killing specific Jungle Creeps that have a crystal floating above their head. Destroying enemy structures also causes crystals to be dropped on the ground which can be picked up. Every hero kill grants the allied team of the killer 1 Crystal.

Your purpose is to help your team advance towards the enemy Core Structure (World Tree for Nature Faction and Necropolis for Nature Faction). Once the structure is destroyed, you win the game.