Opening the book…
Copper and aluminum are not interchangeable at a connection. Aluminum expands more with heat, oxidizes into a resistive film, and creeps under pressure, so aluminum wire on a copper-only terminal loosens and heats over time, a documented cause of house fires. Terminals and connectors are listed for the metals they hold, marked CO/ALR or AL-CU when they accept aluminum. Mixing metals or using the wrong connector makes a high-resistance joint that behaves fine at first and fails months later. The wire's metal has to match the rating of what holds it.
Identify the conductor metal before terminating; aluminum branch wiring is common in certain eras and needs specific handling. Use devices and connectors listed for the metal, CO/ALR receptacles and switches for aluminum, or listed pigtailing connectors to transition aluminum to a copper tail at the device. Apply the specified antioxidant compound where required and torque to spec. Do not land aluminum under a copper-only screw. When you find aluminum branch wiring at receptacles and switches, treat it as a known hazard needing correct devices, not a quick swap.
Aluminum is entirely appropriate and common for larger feeders and service conductors; this rule targets the small-gauge branch-circuit connections that fail. Listed repair methods, approved pigtail connectors, are the accepted fix for aluminum branch wiring. If a connection's metal rating is uncertain, stop and use a listed method or call an electrician experienced with aluminum.