Upgrading Drupal

THe site worked fine in ddev. I updated it via composer and it works just fine.

THen I exported db via CMD and zipped files and uploaded them via filemanager to fresh new folder on live server.

I also imported db in new site. I inserted new database and user in setting.php and tried to open the site. 

THe site does not want to open at all. Nothing helps.

I have updated my website from Drupal 8.9.6 to 9.0.0 (after checking Upgrade status and removing irrelevant/incompatible modules) via the Installatron feature from my provider.
The website looks okay for visitors but when I try to run 'Configuration' as logged in admin, I get this message:

The website encountered an unexpected error. Please try again later.
Error: Call to undefined function config_get_config_directory() in config_requirements() (line 14 of core/modules/config/config.install).

I need to know how to remove the default theme and a module in the database.

When I run update.php I get this error.

I've inherited a production site running Drupal 8.7.11 (which is insecure) and my first target is to upgrade it to the latest version of the Drupal 8 branch (8.9.8).  The site has been built using composer. Naturally, I don't want to mess with the production site, but want to clone the (outdated) code base to a staging site, and then try to do the upgrade there:

I've tried to apply the method described here: https://drupal.stackexchange.com/a/295863/12076 – but it did not work out.

Hi,

I'm trying to update to 8.9.7 without success, even though I never had any issues before.

When I run :

composer outdated "drupal/*"

Here is what I get :

drupal/core                         8.9.6           9.0.7
drupal/core-composer-scaffold       8.9.6           9.0.7
drupal/core-project-message         8.9.6           9.0.7
drupal/core-recommended             8.9.6           9.0.7
drupal/core-vendor-hardening        8.9.6           9.0.7

So I run, as usual :

I upgraded from Drupal 7 to Drupal 9 and the core functionality is working as needed but while finalizing the Drupal 9 configuration, the production Drupal 7 version has had new content, users, categories added. 

How do I now update the Drupal 9 site with the new content that was added to the Drupal 7 production site?