Help a noob learn how Itemization works (Offlane leaning)

tl;dr I'm looking for generalized item explanations, mainly understanding flex/extension picks

Hey all, I'm here looking for general itemization guides, by which I mean guides that explain the purpose of items in a vacuum, not items on specific heroes. Of course, I understand that composition will affect my build, but how can I start A/B testing builds when there are two variables to consider?

For some reference, I'm come from three years of LoL, and a lifetime of gaming. In that game, I started by using blitz or similar programs to give me automated builds. After a while, I started looking up build guides, and began to understand the general purpose of items (i.e. Dark Seal is a greedy snowball item, Kraken Slayer is for armor tanks, Chainsword is for anti-heal and base damage). I'm using the Immortal Faith guides now, but I want to actually understand what I'm doing, and why I'm doing it.

Right now I'm playing mostly Pangolier (yes I know he's really hard blah blah blah), and this is the build that I'm going every game:Start: QB, 2 Sticks, Circlet+Slippers, Tango.Core: Diffusal>Arc Boots>Blink>Shard>Scepter>BasherI get most of these items purposes, but it's my extensions/flexes that confuse me:Boots of Travel, Maelstrom, Abyssal, Shiva's, Lotus Orb, Gleipnir, Mjolnir, Halberd, BKB, Aeon Disk, Eul's.

I get some of these, Boots and Maelstrom+ are good for splitting, Shiva's, Lotus, and Aeon Disk are defensive, and BKB is BKB, but That leaves like half of this stuff totally confusing for me.

Feel free to ask any questions, I'm even willing to do coaching or whatever, if that's the best option to learn this stuff, thanks 🙂