Like any other familiar, you get to pick 2 abilities each day from Familiar Abilities and/or Master abilities. If you qualify for and pick the Spell Battery master ability, then YOU (the master) gain an extra spellslot 3 levels lower than your highest level. Your familiar never gains the ability to cast spells, the best it can do is the Spell Delivery master ability - which says nothing about changing the number of actions required to cast the spell being delivered or Commanding the Familiar as a minion (Meaning it'd normally take all 3 actions on your turn - 2 to cast the spell and 1 to command your familiar, giving it 2 actions to move+touch a target).
https://2e.aonprd.com/Familiars.aspx