The spec
USB-C PD lets two devices negotiate voltage and amperage. 100W = 20V × 5A. Lower-power devices ask for less; the brick obliges.
In practice
A 100W brick will charge a laptop at 100W AND a phone at the phone’s max (e.g. 27W on iPhone 15 Pro). One charger replaces three.
The cable trap
Not all USB-C cables carry 100W. Look for ‘100W’ or ‘5A’ rating on the cable itself. A bargain-bin cable melting at high power is a real failure mode.