Reply To: Customise page number

I’m not familiar with Ventura at all. Regarding running headers this might help: https://helpx.adobe.com/indesign/using/numbering-pages-chapters-sections.html. And master pages: https://helpx.adobe.com/indesign/using/master-pages.html. When you create a new document, Indesign assigns by default an “A” master to all pages. But you customize them according to your needs. In your case you want one master page for the first page of the chapter and a second one for the other pages. You can create as many master pages as needed. As Dwayne says, you just need to become familiar the way Indesign works.