best practices of solid programming

I have a large scale project on ci3 and i want migration to ci4 . We developed a lot of library and try to stand on SOLID principles.

Ci3 is very simple , Libraries on ci3 just like pure PHP programming. we dont use hmvc pattern , just make libraries and used theme.
Where i find example about my custom library , modularization , middlewares , custom middleware and etc on Codeigniter 4?