selectlist | jQuery plugin that replaces the standard HTML multiple | Frontend Framework library

I have a Datatable of JQuery generated at first-page load. I am trying to refresh it according to the selected criteria from the selectlist.

My Datatable initialized first like the following code.

I would first like to take I am a novice at this, my background is asp web forms and I am trying to update my skill set to the latest development language.

I have a view which needs to access multiple modules so I created a viewmodel and pass that to the view. My problem is when I try to access a list from the view model to populate a select (dropdown). I keep getting an error. Below is my viewmodel: -

I have two models with one model (Registration) having the other model (Events) a foreign key. I've created multiple Events objects without a problem, but when I try to create a Registration object, the Events dropdown field is empty.

I have the following code:

Models/Registration.cs

I have Institutes collection. it contains ID, Description, Name, Age etc properties. I can get ID and Description after selection. but how to get other properties (e.g. Name) based on selection. here, I need "Name" property as well.

I have below onchange function written in javascript . I have to get "Name" property value. I dont want to call server logic again -

I'm trying to create something like a combo box in excel. Selecting the Site will automatically select the JobNum and Manager. Site, JobNum, and Manager are from a separate table called master. Currently in the create form that I am trying to do, they are all dropdown list that pulls from the db master table.

Edit - Forgot to mention master table has a simple ID, just an incremental from 1. Added an image for how it looks from the DB

This is how it currently looks

Master table

How I want it to be - selecting site, autofills the bottom two

Master Table from DB showing all columns

Code for select list:

I have installed bootstrap.chosen in my web project and am tring to implement it.

BundleConfig

In the following code of an ASP.NET Core 3.1 MVC project, the Get action method Edit (...) is displaying a dropdown list of zipcodes. It seems that by default, the dropdown displays the first zipcode in the list (i.e. 1301) as the selected value.

If I want to display the selected value from database in the dropdown list, how can I achieve it from action method Edit(...) without creating a view model?

I have a rather complex data model, where zipcodes and city is members in the table Shop, but that table has a many-to-many relationship with product by a conjunction table ProductShop. I'm a beginner so I have been trying to follow this tutorial, but it doesn't get me so far. https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/update-related-data?view=aspnetcore-5.0 This is my Edit get method in the controller:

I have this code. I've watched on other stack overflow posts, but nothing works for me. Do I miss something? I need to make some dropdowns for genre, publisher and author and I couldn't find anything which could help me. I tried viewbag because I found it in many places, but it's not working for me. I don't know if viewbag is for .net mvc core 3.1

Book Model

I know that this question might have been already on this site, but there are some different things in my approach because I use @Html.EditFor and @Html.DropDownList. So I have a drop down list and when I choose the ID there I want to retrieve some information from the DB then populate some fields in the current form. I know that I should use JS but I don't know how.

View: