Galvanized steel was standard for supply lines in older homes. It rusts from the inside, so over decades the opening narrows, pressure drops, the water can come out brown, and eventually the pipe leaks. There is no fixing that from the inside; the pipe has to be replaced.
Polybutylene, a gray plastic supply pipe used in many homes from the late 1970s to the mid 1990s, is known for failing at fittings and becoming brittle, often without warning. Copper usually lasts a long time, but in some homes it develops pinhole leaks, and once several have appeared, more tend to follow.
A repipe replaces the supply lines from the main shutoff to every fixture. Drain lines are a separate system; if your drains are the problem, start with a camera inspection instead. If you have one leak and no other symptoms, start with leak detection and a repair, not a repipe.