Prodigy:
there are 3 types of switches: "Snap" switches, Insulfrog and Electrofrog.
Snap switches are basic ready-to-run track. All the rails are always live. If you have these, your layout will be 100% live.
Insulfrog are the ones where the leg of a switch that isn't selected will be dead - one rail has no current. With these, if you power the outer loop only, there will be no current to the inner loop unless you set a crossover to run between loops. You can isolate sidings with these to park trains or locomotives.
Electrofrog, also called all-rail turnouts: the leg not selected will be dead, but both rails will be the same polarity and it causes shorts if the tracks aren't insulated properly.
Model railroads usually designate the rails North and South. If you use either of the first 2 types of switches, you need to wire to both loops for reliable operation.