NOTE
Embedded Tomcat Auto-Configuration
How Spring Boot creates and customizes an embedded servlet container from dependencies, web application type, and server properties.
This is a historical learning note and may contain outdated or incomplete understanding.
For a servlet web application with embedded Tomcat dependencies, Boot provides a servlet web-server factory and applies server/application customizations. During context startup, that factory creates/starts the embedded container and deploys the application context into it.
Properties control common server settings while customizer/factory beans provide programmatic extension points.
Container factory class names and lifecycle integration change across Boot/Tomcat generations. Keep the stable model: web context → conditional server factory → property/customizer application → embedded server lifecycle.