can someone explain how to develop CRON Jobs and schedule reports ?

Gotcha. So I generally relied on my DevOps team to support Cron jobs... so I don't know how much help I can be in setting something up... (I could probably help with the API calls themselves if you need support there).

But for the Above screenshots... in Workspace... when you open up any saved workspace report, you can use the Workspace menu item:

Share > Send file on schedule

This will open a dialogue where you can set up the schedule... if you need to modify that schedule, you can go to the Main Menu:
Components > Scheduled Projects

For Excel Report Builder, this is a plugin that you can add to your Excel (windows only, unless you have CJA and are using the CJA Report Builder). This is basically an interface to create direct data pulls into Excel (https://experienceleague.adobe.com/docs/analytics/analyze/report-builder/home.html)

One of the options in the interface is "schedule the report". These reports will actually show in the old "Scheduled Reports" interface in Adobe: Components > All Components

Then you should find the option in the list.

This might work for your "custom reports", depending on what you are trying to do (and if you are building them in Excel).. 

Hopefully this gives you a start... sorry I can't help more with setting up a Cron Job; but if you are setting one up, I know that it should be set up from a server. Either a prod one, or something in your company's infrastructure rather than a personal desktop so that is it on a cloud infrastructure or has backup power, etc.... this is another reason to have DevOps involved... 

Good Luck!