I was very lost in the transition from D7 to 8+. I was one of those people who "learned Drupal" (from 5 to 7), and so when version 8 switched to OOP, a lot of my Drupal knowledge no longer applied, and I didn't have a deep background in PHP to help me out.
If you're in a similar situation, the book Drupal 8/9/10 Module Development by Daniel Sipos helped me to see both at a high and low level how things are done now. I would consider myself a hobbyist programmer and I own every book Amazon sells with Drupal in the title, and this is the one that has helped me the most in just understanding what the heck Drupal is doing, particularly all the "new" (as of D8) classes and services stuff.
The Drupalize.me screencasts are also very good, although they cost significantly more than a dev book.
Another thing that can help a lot is having an IDE set up properly (code completion for Drupal classes, automatic checks for Drupal coding standards, maybe PHPStan). I use PHPStorm which costs a little money but prevents a huge number of mistakes. For example, if you paste a code example from this site in there without using the class, the Problems pane will warn you that the class is unknown, and you can right-click the class name to import it automatically.
VSCode is also good and free but maybe a bit harder to configure (it was harder for me anyway, although that could be because I'm used to PHPStorm). Of course, if you're like me, setting up the IDE is also a learning experience, but there are docs on Drupal.org and if you choose PHPStorm you get support from real engineers with your subscription.
Also, using a local dev environment like ddev or lando with the IDE will really raise your skill level. Specifically, you need to have xdebug available in your dev environment and know how to use it. With Drupal 7, most of them I was able to get by without a debugger because I could follow the code as the steps occurred more or less in order. With D8+'s OOP, that's impossible; you have to have a debugger.
