Most Minecraft servers feature infinite worlds of procedurally generated terrain. The problems with this are many: chunks sprawl forever, economies inflate under the burden of infinite resources, griefing compounds, and over time the map becomes a graveyard of abandoned bases as old players leave.
Ouroboros does it better.
Each Age is a refresh of Ouroboros SMP's finite world, with new lore, new events, new themes. Right now (Age 0, the Primordial) the border sits at ±10,000 blocks from center on X and Z, 20,000 edge to edge. That number's locked for Age 0; later Ages might move it, haven't decided yet.
When one Age passes to another, only the current, active communities remain. Old and abandoned groups pass away, their memory lost to the ages.
The Wildlands isn't a zone on this map, it's a separate server. You reach it by traversing into the Veil, the fog bank in the center of Selvath, and it loads you onto that Age's Wildlands. Each Age's Wildlands is hand-built for that Age specifically, tied into the wider Ouroboros lore, and it's what actually drives the story forward.
How the Turning actually gets triggered is still being worked out. Whatever it ends up being, expect it to be marked by a buildup in the world, not something that just happens with no warning.
See The Ages for the current Age ladder and where we are in the loop.