On Intel-based Macs, press ⌘R during early boot (before the startup chime) to enter recovery mode.

On Apple Silicon-based Macs, press and hold the power button until you see "Loading startup options" appear below the Apple logo.

Mounting the root volume read/write

Under normal operation the root volume is mounted in a read-only state, preventing tampering. It is mounted read/write in recovery mode, below /Volumes (check mount).