As far as I know, here's the timeline.
Viktor creates the Battlecast robots for [insert villainous diabolical plan here].
Heimer and some other people (probably Jayce too) create the Mechas to counter them.
Viktor wins by hacking Mecha-Xer0 Sion. Probably not why he wins, but he wins in the end.
Viktor creates new cybernetically enhanced soldiers-- PROJECT Overdrive and Disruption. Perhaps used as a trump card to beat Jayce/Heimer? Being that they're efficient killers with magic/tech enhanced powers.
PROJECT Zed and Katarina remain as his loyal puppets.
The others (not sure about Yasuo, but maybe him too) rebel, thus launching the Rebellion (Ashe, Ekko, Leona, Fiora, Yi, Lucian).
I'm assuming Viktor's main Battlecast line was mass-produced, considering that the Vel'Koz in PROJECT: Kat's splash-art could be monitoring the skies sort of like the blimps in Batman's Gotham City. Or that Vel'Koz could be the actual Battlecast Vel'Koz, just with probable upgrades.
Keep in mind that Viktor only controls Ionia, called Neo-Ionia in the PROJECT timeline, and probably Zaun. Not confirmed if Piltover, but if he won against Jayce, Piltover too.
This is as far as we've gotten in terms of information. Then there's a huge gap. The Program skins are part of the timeline, but in the distant future (probably a good 75 years in the future), as they are purely robotic, rather than cybernetically enhanced like the PROJECT skins. It's unclear if Pulsefire is connected, or part of it's own universe, but in the event that it IS connected, it'd have to be at least 100-200 years in the future, based on the technology involved.
So there's this HUGE chunk of history missing from where we are with the current PROJECT skins to the Program skins. It's not confirmed whether or not the Rebellion overthrew Viktor's reign.