It’s very useful to have access to another Zoho account in your organization when first learning about all these sharing and user permissions.
Here, I log into Sergei’s account (another employee at Affluence Financial Planning). As you can see, he, after logging into Zoho Analytics, can only see the one dashboard I shared with him. Also, in the underlying data, he can only see what I allowed him to.
If you want someone to collaborate with you on reports and dashboards, allow them into your workspace as an admin and not a user. Or, leave them as a user and only give them access to the few tables of data they need to build their reports.