How do you unlock Poppi's Final Form?

Solution 1:

To get Poppi's Final Form (qt pi), you'll have to travel to the Leftherian Archipelago (specifically Fonsett Village Arch) and walk to a Heart-to-Heart. You can find it by going down the stairs and to the right, across a bridge and onto the next landmass.

Afterwards, you can find the Heart-to-Heart by around the left side of that particular island. It's located here:

After viewing this, head back to Tora's House, and you'll get the "Powered up Poppi" quest, which requires you to get 10 Golden Cogs by salvaging in Temperantia.

Afterwards you'll need to get 5 Cubic Diamonds from Mor Ardain (gathered), 12 Ambler Stigmas from Land of Morytha (gathered), and a Genbu-Weave Cloth from Tantal (purchased).

You'll then need to talk with an npc in Argentum. This will require Forestry 5, Ancient Wisdom 3. You'll also fight a level 50 monster.

For the final stretch you need to either do a Merc Mission a few times or get a lot of materials.