you can use composer create-project with the plainkit. that logic is something like a git clone.
as far as i understand the code composer installer package of kirby its only changing/renaming the vendor folder. that has nothing to do with the complete plainkit repo.
edited: it should be used with getkirby/cms not with the other ones.
I tried the create-project solution at first but I wasn’t happy with it because of the location of the composer.json file. I like having it at root with all other config files I have for the project. Also, it simpler in CLI to avoid running cd everytime.
The Plainkit is just a template for your own project. You should not require it with Composer but instead install it one time (with create-project) and then modify it according to your needs.
If you’d like to have the composer.json outside of the web root, you can configure the installation like this: