Module development and code questions

I have nodes with puzzles. I store extra info on the puzzle (say puzzle_sol) in a different table. I add this extra info to node in 'hook_node_storage_load' in my module. I then attach a special library to the page in page_attachments hook. There I use the above extra info to decide whether to attach the library or not. (if puzzle_sol was added, then attach the library ... else not)

This whole strategy is working without absolutely any issue.

Now the problem.

I am currently trying to patch a module so that you can use the local oembed thumbnail to display as a preview.

Currently the direct url to the youtube thumbnail is used but that is, of course, not allowed by DSGVO/GDPR cancer.

My idea was to use the hook and load the thumbnail there. But OEmbed::getLocalThumbnailUri is a protected function.

Any ideas hints or tips? Any contrib doing this where i could read some code? Thank you.

function hook_preprocess_media_oembed_iframe(array &$variables) {

Background:

I am using hook_node_access_records() , and hook_node_grants() in a custom module for node acces control and it Works perfectly

I am wondering if there would be something similar for media entities??

Thanks in advance for your advices.

I am still struggling to understand how to use dependecy injection correctly.

(This is all from the contributed extension Notify)

It has a a service class named "Notify".

In it, I am doing a lot of direct calls to global service containers. Example:

$nodes = \Drupal::entityTypeManager()->getStorage('node')->loadMultiple();

This works (i.e. it gets the nodes), but I am getting warned by PHPstan:

Heya! I'm working on a project for a client who is using Drupal 7. Among other things, we want to perform an audit of the content currently stored in their Drupal instance. Specifically, I want to create a list of every page / link on the site and any related files / downloads that Drupal is managing for them. 

Looking at the APIs, I honestly have no idea where to start here. I am also digging around the database and, while I see the nodes and various field_ tables that seem to have the content stored in them, I see no obvious way they're connected.