You can Deconstruct dropped Items at the Smithing Workstation.

The EXP you get towards the Skilline depends on the Quality, the Level and if the Item is "Intricate".

The higher the Level of the Item, the more EXP you will get.

This works for Smithing, Clothing, Woodworking and Jewelrycrafting.
Echanting works similar, but there is no "intricate" glyphs.
Provisioning and Alchemie are both Leveled by Crafting food and Potions

Cloth, Wood, Metal, Enchanting, and Jewelry are best leveled by deconning mob loot. Alchemy and Provisioning level best bu making stuff.

Now, as you level your Craft lines, you need to invest Skill Points into the material passives to be able to craft higher level items. Yes, you can eat up 50 to 70 skill points just in material passives.

Note, while leveling to CP160, just use mob drops