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There is currently no way to create a dynamic navigation dropdown menu (or list, or other content web part) to show all the sites that any given user has access to. It's simple to know who a site can be accessed by, but it's complicated to know which sites a user can access.
Perhaps you want each of your users to simply know which sites they can access.
The first method is through the SharePoint start page. The SharePoint start page (https://YourDomain.sharepoint.com/_layouts/15/sharepoint.aspx) displays Frequent, Recent, and following sites. This helps the user to quickly navigate to his most recently visited sites, but the downside is that sometimes only some of the sites available to the user are shown here.
Another way is to use SharePoint Search to get a list of all the sites a user can access.
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