Analog and mixed-signal design depends on clear system modeling concepts. A system is not only a group of blocks; it is a connected structure made of components, ports, nodes, branches, nets, and signals. Each connection carries meaning, and each modeling choice affects how behavior is interpreted during simulation. Clear definitions of hierarchy, leaf components, signal continuity, analog ports, digital ports, and mixed ports provide the foundation for describing complex designs in a controlled way. Verilog-A gives analog designers a high-level way to represent behavior and structure. A .va model can describe analog functionality with Verilog-style syntax, interface declarations, parameters, local variables, structural statements, and behavioral statements inside an analog block. Proper coding rules matter: case sensitivity, include directives, module boundaries, semicolons, port disciplines, and matching schematic symbols all affect whether a model can be used correctly in a design environment. Electrical simulation connects netlists, component models, primitive definitions, and numerical solving. The netlist defines the network structure, while models define how connected elem...