How do I clone an existing site? I used the following steps for a D8 site:
 git clone 
 composer update (get the packages)
 make a database
 make files folder and settings.php writable
navigate to d8.localhost.install.php (host made and site enabled)

Somehow this did not work do I miss something?
I think it can depend a little on what your goal for the "clone" is.

When we want to make clones of a live site for development or testing or whatever very little works as well as just making an image of the whole server, spinning it up somewhere else and modifying the allowed domains to account for the new IP address.

If you want specific help about some part of the cloning process then perhaps we need more information about what s going wrong; error messages, WOD etc. I notice in your post you mention making the database but not importing the old database, was that step taken?