Long haul power transmission is almost always 3 phase AC in the US - it can be done using either 3 or 4 wires. It tends to better distribute loads and requires smaller conductors for the same power transmission level.
Split phase is almost always the last mile to residential customers as it’s simpler and easier to reason about (less likely to have severely unbalanced phases too) and requires fewer conductors.
Split phase is almost always the last mile to residential customers as it’s simpler and easier to reason about (less likely to have severely unbalanced phases too) and requires fewer conductors.