Where to start?

Hello, For many years I have managed a few Drupal sites from the UI backend. I would like to update my skills and also follow the software development and would like to start extending my Drupal installation through Composer.

My hosting provider allows me to deploy a new site with Installatron. That's really convenient because it installs a new site with Drupal, creating the database and such automatically.

However as soon as I install a module through composer (for example webforms) then immediately I loose access to my admin webpages. The newly installed site's main page remains working. Just when I try to access the admin backend I get an error message: the website encountered an unexpected error. please try again later

My question now is: am I on the right track? I have read that when you manage modules and dependencies through composer you should not install modules manually through the UI. But on a fresh install?

I also do believe that the Installatron mechanism uses composer because I see a composer.json in the site's directory.

Any guidance on wether I am on the right track or should go down a different road would be much appreciated

With friendly greetings,
Rob Bastiaansen