Generally both are part of SAP’s SCM box but can be implemented independently.
APO is used for advanced planning of demand(generally finished goods, though other material planning is quite possible), supply (various manufacturing plants to various DCs, Inter DCs, Various DCs to customer location, plants to customer locations etc), detailed production planning and scheduling, ATP checking at whole enterprise level , Truck load planning etc. APO is generally used for big enterprises having multiple plants, multiple DCs, a number of suppliers and customers etc.
You can view APO as a planning tool(demand forecast to production and some level of distribution) whereas SNC is a both planning(small extent) & execution tool mainly used for collaboration perspective (with external business partners such as Suppliers <-> Plants <-> customers and Plants<->external manufacturers).
Both tools have different objectives, they work very differently and both are required for enterprises to increase their KPIs in supply chain.