How to check your Mac is booting in Full Security, and how to read its log to verify all the key steps involved in that process.
Startup Security Utility
Systematic and thorough account of the structure and function of bootable external disks and dual-boot systems from High Sierra to Sequoia, and how to diagnose their problems.
Going deeper into Recovery mode, by setting the correct keyboard, sharing the Data volume with another Mac, using full features in Disk Utility and Safari, and avoiding repairing Home permissions.
Explains the three different settings, including Permissive Security, why you might need to reduce security, and how to do so using Startup Security Utility.
The Eclectic Light Company 