Happy birthday to Drupal! On January 14th, Drupal turned 25. Dries wrote up a nice reflection on those 25 years. In that, he wrote this

6. Nobody fully understands Drupal anymore, including me

After 25 years and tens of thousands of contributors, Drupal has grown beyond any single person's understanding. I also google Drupal's documentation. I'm strangely proud of that, because it's how I know it has become something bigger than any one of us.

Don't feel like you need to know it all. Grasp the fundamentals, Google the rest.