So, to upgrade Poppi, you need technical manuals to learn how to make parts for her. I've really only found one in the old factory in Mor Ardain. Trying to find all of them so I can make Tora a more versatile character, instead of just a tank.
It seems there are only 5 technical manuals to find. The locations of each are as follows:
Nimble Nopon Get Girls
Uraya - Head - Greatmaw Rapids - if I remember right, it's in the water.
Artificial Blade Report
Mor Ardain - Old Factory - Work Observation Tower - it's in a chest on a ledge and around a corner. It's visible from the floor.
Ether R&D Revolution
Kingdom of Tantal - Theoscaldia Palace - Hall of Reception - Storage Room - it's in a chest
Robolab Club Quarterly
Land of Morytha - Fallen Skyscraper - Highway 8 - go up the building, walk south along the highway, and then west across the connecting highway
Hypertech Made Easy
World Tree - Lower Level - Sky Bridge - in a chest guarded by a unique, requiring, I think, 6 fortitude and X lockpicking
Credits to this reddit post for helping me find the last 2, and reminding me where I found the first one.