Your best option is to subscribe to drupalize.me and work through their content. (I'm not affiliated) Unfortunately the changes over the years with Drupal mean

Your best option is to subscribe to drupalize.me and work through their content. (I'm not affiliated) Unfortunately the changes over the years with Drupal meant you can find guides online but they may be outdated. There is also a book "Learning Drupal 9 as a framework" written by Stef Van Looveran. He appears to keep it updated and while it isn't a complete resource, it could be worth a look.