Augments slots are automatically added to equippable items (armor shells, generators/focii, earpieces, implants, weapons, and relics) that were crafted with a critical success, which is rare.
Reverse engineering equippable items will give you Augmentation Slot Components that you need for crafting Augments Kits, which can be used at an Item Modification Station (AKA "Augment Table") to add an augment slot to any equippable item that doesn't already have one.
Augments themselves are separate items that go into the slot and give a single primary or secondary stat in addition to Endurance — either +18 stat +12 End or +18 End +12 stat. There are currently no Expertise augments in-game although they are in the assets.
Augmentation Kit schematics are learned from the Crew Skill trainer. Augment schematics come from Slicing, specifically Tech Parts missions. Both types of items can only be crafted by Synthweavers, Armormechs, and Armstechs; they all craft the kits and have the augments split between them.