Can I chage the homepage design by replacing the background with another image or slider on Experthive?
if I want to use theplusblocks builder to change the design, should I create a child theme to keep the site’s code intact or does it only make style changes?
Yes, it’s possible to change the background color in Appearance/Customize/Colors. If you mean customizing the header layout to set a full-size background like this https://jobhive.hivepress.io/ then this would require CSS tweaks. If there’s a third-party slider (e.g. added via a plugin) you can add it to the header area by editing the Home page, and disable built-in header image stack in Appearance/Customize/Homepage.
I checked it and it seems like a collection of blocks for the native WP editor (Gutenberg) so if you just use their blocks in the editor creating a child theme or code customizations are not required.