Module development and code questions

Hi everyone !

I have a custom module, defining a custom entity.

I have follow this guide 
https://www.drupal.org/docs/8/api/entity-api/creating-a-custom-content-entity#s-building-a-bundle-less-content-entity-type-in-drupal-8

The entity works well, I can do all CRUD operation, view ...

I use it with the json:api, works great for the entity, my problem is with the fields.

On our site, we are getting a log message with a Warning referencing code in a custom module:

Hi,

I'm trying to contribute to a module for the first time, but my patch is always failing to apply, can anyone explain to me why?

Here's the issue I've created : https://www.drupal.org/project/paragraphs/issues/3130327

I've followed the instructions at https://www.drupal.org/project/paragraphs/git-instructions

I did :

When adding a new field to an entity type, what determine the list of the available entities types that can be referenced? Where and how can I change this list?

I want to reference Paragraphs (for a specific reason I need to use a reference field an not a reference revision field) and I can't select "Paragraph" as a type to be referenced (I could before an update). I need to manually edit the field configuration to make it work.

Hi,

For my website, I want to give users an option to save their selections before logging out. Once they click the logout button, I want to open a modal pop up which gives them 2 choices, Save or Cancel.

If they select Save, then their selections would be saved in the database.

How could I do so in drupal? If there are any examples online, please let me know.

Thanks

I am trying to import an xlsx import into my site. I already have the feed setup, but there isn't a way to build a composite unique field (like for example, if any value in column 1 and any value in column 2 together (but not separately) exist, don't import. Is there a way I can do this? I'm trying to avoid building a custom parser if I can use built in feeds to do it.