Module development and code questions

We have a node type called: MEETING On Meeting we have a user entity reference called: PARTICIPANTS I create several MEETING's and reference 2 users: USER1 AND USER2

Now I want to build a dynamic view we can use URL arguments in contextual filters to show all meetings with USER1 and USER2

I create a view called VIEWMEETINGS I set PARTICIPANTS field as the contextual filter and check "Allow multiple values"

view path settings are MYSITE/VIEWPARTICIPANTS/%

I am using Drupal Commerce and looking for a way to exempt some items from general tax.

I have tried a couple of modules to achieve this for instance Commerce Product Tax, Commerce Product Tax Conditions but they don't seems to be giving an option of selecting which items are tax free.

At product creation level I am expecting an option to select which items are tax exempt.

I created a custom module for Drupal 8 and Drupal Commerce. It is used to display a link "Accept the conditions of sale" in the purchase funnel during an order.

This link displays a modal window with the rendering of a page.

I have created a "terms and conditions" display mode in my store types. I want the link to show the store's rendered "terms and conditions" entity in a modal window.

So I want to make the entity store instead of showing a page in the modal link.

How to do this ?

Here is my code :

I want an entity where I can store various bits of data (using fields) and link that data to multiple page nodes. But that data should not be visible to anyone - it is more for adding and displaying data programmatically or via Views. What's the best way to go about this?

My constraints are being called from a hook_entity_type_alter

Do you now how I can build complex table in Drupal? I want to get something like this (in upload picture). I have nodes with fields: nid, code, name, date, sum. Thanks! 

https://drive.google.com/file/d/1BNpHNYMoVrOI5CMGaQuMj50t12GvGkdr/view?u...