It took me a while to get my head around.
This bit you know. Play the game to get ether crystals (either in the stage, or from chests) and Poppiswap parts (from chests, including the big chest at the end). Extra stages get unlocked as the game goes on, which also makes more parts available (although 99% of the parts are available on any stage, apparently - so once L2 is unlocked, L1 will also start yielding new items, etc).
Ether Crystals (top right) - get these from playing Tiger Tiger, and you can also turn unwanted Poppi parts into Crystals, which took me ages to realise. These are your currency for all things Poppi - you can use them to synthesise parts (more on that later) or unlock upgrade slots.
The energy meter is that thing on the right (100/350). Each Poppi component uses some energy (the top number), and your limited by your maximum (the bottom number). This is basically like base building in X-Com or Starcraft.
Energy Converter - upgrading this increases your energy cap, allowing more/better upgrades. It's cheap at first, but gets very expensive.
Role CPU - this defines what kind of Blade Poppi is. There are a few different variants of TNK, HLR and ATK. In theory you can make any Poppi be anything, but in practice their passives on the affinity chart kind of point you in the right direction (face tank for alpha, dodge tank for second form, attacker for third). Higher levels of mods give better stat boosts but need more energy.
Element Core - what I just changed to Ice for Poppi Alpha (this isn't my screenshot). Not all Poppis can be all elements, for some reason. Dark/Light elements draw more energy than the rest.
Arts Cards - these are equivalent to blade arts, the ones that activate from time to time on your normal blades, e.g. back attack, hasten special recharging. You have to do a one-off payment of ether crystals to unlock each card slot. Different arts cards (and higher levels) draw different amounts of energy.
Skill Ram / Skill Upgrade - these are equivalent to Battle Skills on your normal blades. All three of these slots do the same thing, it's just that you get Skill Ram unlocked for free and have to use Ether Crystals to unlock the other two. The third one is crazy expensive. Again, different skills/tiers draw different amounts of energy.
Specials Enhancing RAM - these are the extra effects of your L1/2/3 specials - you know, the nodes on Poppi's affinity chart that say "Effect determined by Poppiswap". You have to pay Ether Crystals to unlock these, in order. Different tiers draw different energy.
Annoyingly, nothing is shared between Poppi forms. You'll have to boost each one's energy caps and unlock their slots etc.
And you thought the other bit was badly explained! This view defaults to Create Parts, which lets you use Ether Crystals to make new parts, but you can hit Y to toggle to Crystallize Parts (shown here) where you can effectively "sell" any unwanted parts to get more ether crystals. This is vital, as you'll get quite a bit of junk via Tiger Tiger, and this means it isn't wasted. It took me ages to work out you could do that.
On the create side it's pretty straightforward, but there is one big unexplained thing - Poppiswap Manuals. These are key items found throughout the game which increase the number of parts you can make. I've found three so far - one at the mouth of Uraya, one in the old factory in Mor Ardain, and one in Tantal (which finally gave me the ability to make elemental cores). The names vary from "Artifical Blade Report" to "Nifty Nopon Get Girls" - I think the idea is that Tora's father/grandfather left them behind.
Finding manuals will unlock new parts or higher tiers of existing parts. Note that you can get things from Tiger Tiger which you don't have the manual to synthesise yet - if you find one you can make extra ones, presumably via reverse engineering. As mentioned above, there are a few parts which can only be got in Tiger Tiger, but the Dark/Light Element Cores and the Master Mods (both Leve 5 exclusives) seem to the only ones anyone cares about.
I think that's about it. Apparently if you're willing to grind Tiger Tiger, then Poppi is the most stupidly overpowered character in the game. Right now I'm on Ch6 and largely using the other tank, mostly as she can have three different elements rather than just the two. Later on, I'll mix things up a bit.