Custom table how to retrieve data or edit the CRUD functionality of the custom table

You can use alternative forms for custom table to add your custom HTML (Custom Layout). Please check below URLs.

https://docs.kentico.com/k12/developing-websites/defining-website-content-structure/managing-custom-tables/creating-alternative-editing-forms-for-custom-tables

Custom table input Edit web part in Kentico Marketplace - https://devnet.kentico.com/marketplace/web-parts/custom-table-input-edit

CRUD web part example - https://devnet.kentico.com/questions/crud-web-part-example

Custom table data using API - https://docs.kentico.com/api12/content-management/custom-table-data

Generating classes for Kentico objects - https://docs.kentico.com/k12/developing-websites/generating-classes-for-kentico-objects

If you're looking to load data from your custom table using only javascript (i.e. no C#), you can use our REST service: https://docs.kentico.com/k11/integrating-3rd-party-systems/kentico-rest-service/getting-data-using-rest

As Vasu mentioned, you can use the Code tab of the custom table to generate the default code for the class, then modify this code to meet your needs.

Hi can't edit the default code of the code tab of my custom table

You can click the Save code button to create physical code files in your project, then edit those.