Module development and code questions

In looking at the Documentation in a REST POST of a Comment I saw 

https://www.drupal.org/docs/drupal-apis/creating-entities-with-the-core-...

Hello

I'm new to Drupal and trying to learn how it works. I've created an easy Hello World module, but when I test it I get the message "RuntimeException: Failed to start the session because headers have already been sent".

I've tried to search for it, read several possible questions, but couldn't solve it, most of the times because my little knowledge of Drupal.

I've found this solution, but really I don't know where to do it.

My Drupal 9 platform has a conflict between the Iubenda Integration module and Doubleclick for Publishers (which uses Google Tags).

Iubenda Integration for some reason changes the type attribute of some <script> elements to type="text/plain." This means the JavaScript inside does not run. In this case, it is where the googletag variable is being declared, resulting in multiple errors that say "Uncaught ReferenceError: googeltag is not defined."

Hi everyone, 

I am busy developing a drupal module that pulls data from my companies product and displays it, I have the module working in D7 but during the upgrade process for D9 & 10 I want to make the resulting data easier to theme or plug into existing drupal websites, classically I have just been making routes and serving the template with the occasional theme override

What I would like to know is could I replace that style of working with blocks to make it less dependant on having the theme set up perfectly?

I have a product type with its multiple product variations using 2 or 3 product attributes like color, size, etc. Here is what I am trying to achieve:

On a product detail page whenever the user changes a product variation by selecting a product attribute value i want to show a message for the selected product variation (like Max. purchasable quantity of this variation ). I am using ProductVariationAjaxChangeEvent to fire a ajax command that will alter the markup of that page and add a message somewhere on the page. here is my event subscriber code

I'm getting this error while trying to run Cron. unfortunatly I have no idea where to look anymore. I've seen simular issues but not for commerce recurring. Does anyone have an idea of what could cause this issue? And how can I fix it?

PHP 8.0.27
D9.5.0
Commerce Recurring 8.x-1.0-rc1

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