In something like Ironsworn, in Session 0, the group decides the Truths of the setting - a series of ten or fifteen overall themes of the setting - how heavily or sparsely populated is the area, how long have humans lived there, how common are monsters, how common is magic - that sort of thing. Pretty high altitude stuff. Once that's decided, you make characters and start play. A couple of die rolls and you have a starting settlement typically with some sort of problem. From there, everything is created by the group in play. You travel out to explore - make an Undertake a Journey roll. What did you find? Well, one of the players around the table can answer that. Once that's established, play continues. Within a handful of sessions, you have factions, plots, and all sorts of fun stuff going on.
Put it this way. We played Ironsworn for about a dozen sessions. Within that time, we established a cosmology, brought in two or three major plotlines and a half a dozen sub-plot stories; I have no idea now how many NPC's, I'd have to check my notes, half a dozen or so communities, and I'm sure I'm forgetting other stuff. It was an absolute blast. Prep? Did none. Zero. Zip. Nada. All done in play.
Six months of play? We'd have epic storylines by that point. Players would likely be on the third or fourth generation of characters. I'd actually really like to give a spin when we finish up our current D&D campaign - there's some additional books I'd love to try out. Add in some method for hexploration - there's a flower system (and that's not the right name, but, that's what comes to mind) that I've found that would be fantastic for this. Six months? Good grief, I'd have a setting with Ptolus levels of detail.