Drupal 8.x

What is the general approach in determining what part of the Drupal structure Your_Module should ‘contact’? How do I find out where is the code which I’m trying to influence? I never seem to be able to quickly pinpoint which part of the Drupal universe I’m trying to influence.

Suppose, for instance, I want to ‘grab’ the body text of a content type which has just been added or edited. Where happens what? And how to find out? This undoubtedly sounds like a basic question. It is. That’s why I have the courage of asking it.

In the past, I used to manage a Drupal 8 site. Now I wanted to reuse that drupal site. I uploaded the data on the site. it has given me the following error message. Could anyone please guide me on how to handle these errors so that I can complete the installation?

File system

Hi team

I have an issue, i would like to know how to set cache to a json response,

I've created this controller and i've tried this:
 

On drupal 8/9 I would like to:

1. upload files to the private folder 
2. allow only authenticated users to view the file with the direct link

Ideally, I would like to control that access by role but i'll settle for 2.

After a lot of googling and reading documentation, I am getting the impression this is not possible in Drupal 8. 

Could someone give me a definitive answer on this? I always assumed the above was one of the basic purposes of private files but... clearly not. 

Here's what's happening.

When i click to view a user profile the link looks like for example:

website.com/user

Then when i click the home button it will go to

website.com/user/home

And since the link doesn't exist i get a page not found error when it should go back to the home page

website.com

using drupal 8...any help would be appreciated

On a separate note, haven't upgraded to drupal 9, because it uses mysql server version 5.7 which is not supported by