Hello,
First of all thanks for choosing our Theme, we are glad to be you in WoodMart WordPress family :-)
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You can add any custom code in main theme but the changes will lose whenever you update the theme so it’s better to use child theme.
You do not need to download the child theme from somewhere. Because we already made it for you (you will found in theme package downloaded from themeforest). Here is the link that demonstrate how it works: https://developer.wordpress.org/themes/advanced-topics/child-themes/
Once you activate the child theme. You can do any customization inside it and they will not lose even after updating the theme.
Best Regards.