The Outpost structure rarely spawns in plains and savannah biomes. This structure consists of a 7-block-tall bedrock pillar with a blue core near the top. Right-clicking on this core with the appropriate resources in your inventory will place the resources on the pillar, and enable further construction. Once all resources have been deposited, the Outpost Center will be completed, and 3 Build Pads will spawn around the completed structure.
Once you have enough resources, you can upgrade the Outpost Center to Tier 2; this will spawn two Build Pads for smaller buildings on the south side of the outpost.