Composer question

You have read the documentation about it I take it? App Starter, it's the same as Adding CodeIgniter4 to an Existing Project. But without the need to copy the folder from CodeIgniter4.

So why use Composer?
You can update the framework automatically, no need to download a .zip file, unpack it etc.

Why do you have /vendor/...?
CodeIgniter4 itself are placed in the vendor folder as with all Composer packages. So that it can be updated outside your project. But you still need to have your own application somewhere, and that's what app, public, tests, writable etc comes in. Your develop your app in app, and NEVER touch the content in vendor.

If you download the zip folder/framework itself, you will find a system folder (instead of vendor). Difference here are that vendor contains ALL folders, not just the system. So when you are upgrading your application you can copy those files from the vendor folder (if a config file have changed) and replace yours. Your project also will have a smaller footprint, as you don't need to store the vendor folder yourself, that can always be re-downloaded with composer.