How, what, where?

for front end you can use Ci4 template view in conjunction with Bootstrap. The template is very similar the way you use them to Laravel blade. You just have one template view that contains all the links to bootstrap css, contains all the header and footer stuff , js scripts just before the end of the body tag. All views which just have the relevant content simply "extend" the main template. By leveraging bootstrap you also have access to flexbox

https://codeigniter4.github.io/userguide...youts.html

CMS CI4 A CMS system, runs out of the box written on top of CI4

Arch Book  CodeIgniter4 on Apache(pages 92-114) 

CI4-CMS Github