Is there any way to move the side nav of subsite into the top nav?
Any suggestions or methods that can easily understand?
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The navigation on the left hand-side is called Quick Launch Navigation, and SharePoint have another type of navigation called Global Navigation which is placed on the top bar.
Simply we cannot move one to another.
For what we can do to customize navigation without code, please check this.
By default left navigation is enabled for team site and it is disabled for communication site - the communication site has the top navigation.
In modern SharePoint online, the recommendation for navigation is Mega Menu navigation whether it is a team site or communication site.
Refer to the below articles for Mega Menu configuration :