The “Main” World where you start has a global Item Level attached to it that rises as you play the game. Slaying bosses, completing maps, doing quests, all have an effect on the global item level.
Most of the items in Path of Terraria can be dropped as long as their minimum item level is met. Each item has their own Item Level requirement. Check the page on these items for information on their Minimum Level Required.
All items that are dropped will have the Item Level from the world they were dropped in.
Maps have their own Item Level attached and do not have the global item level assigned or added to them.
Right now none of the Path of Terraria specific items have the ability to be crafted. This is intended to be changed in the future as we flush out the systems.