Module development and code questions

Hello and hope this is the appropriate forum section.   I searched for my key words but no results.   I am using Drupal 9

I may be making heavy weather of this.   I have a node with several fields one of which I want to be hidden initially, with a tick box or radio button or similar that the user can click on to reveal the field's contents.

I have looked for Drupal modules to support this functionality for example Rules, but none seems a good fit.   So diagrammatically:

Field 1 (always on display)

Hi team

I've created a view using REST Export

I'm using an url like this to get some items that has state 356 and table bzn:

http://localhost/items?status=1&state=356&table=ZBN

And this works good, but i would like to add one or two values to table, something like this:

http://localhost/items?status=1&state=356&table=ZBN,LXX

Hello,
Can anybody tell me why I keep getting the messages in the "Subject" line displayed on every page? Is there some definition somewhere? I am on Drupal 9.2.7, but this also happens on earlier D9 versions. 
Thanks for any hint
H. Stoellinger

I have the below data structure:

Hi, I am attempting to learn Drupal by working my way through Drupal 8: enterprise web development : harness the power of Drupal 8 to create enterprise-grade, highly scalable websites by Nick Abbott. In the chapter about creating custom modules, we are supposed to create and install a custom module whose output is routed to the page /mypage. I followed along exactly, triple-checking the code, but it doesn't work. Instead of getting the output of my module, I get a Page not found error. I checked and there are no published erata for this chapter of the book.

I'm porting my first module from Drupal 7 to Drupal 8 and 9, the module itself works, but now I'm stuck on the hook_install. The purpose of my call to hook_install is to create some directories in the public files directory as well as copy some files into them.

In Drupal 7 I used "file_prepare_directory" to create the needed directories. In Drupal 8 this still works, but is advised against since it's deprecated, But it's removed from Drupal 9.x. So I've found it can be replaced by