Design patterns

What is singleton ?

https://devdocs.magento.com/guides/v2.4/extension-dev-guide/factories.html

What is Observer ?

https://devdocs.magento.com/guides/v2.4/extension-dev-guide/events-and-observers.html

What is dependency injection ?

https://devdocs.magento.com/guides/v2.4/extension-dev-guide/depend-inj.html#:~:text=Magento%202%20uses%20Dependency%20Injection,Mage%20class%20in%20Magento%201.&text=Dependency%20Injection%20is%20a%20design,B%20that%20supplies%20those%20dependencies.

What is Difference beetwen ResourceModel and Model ?

Magento 2: Models, Resource Models, and Collections

What is diference beetween cache:flush and cache clean ?