External users’ limitations inside sharepoint online sites, regarding cross-site querying and search

I was reading this article about External users in SharePoint online:-

https://blogs.technet.microsoft.com/lystavlen/2013/04/14/understanding-external-users-in-sharepoint-online/

and the author mentioned the following limitation which external users will face inside sharepoint online:-

By default, external users cannot access the Search Center and will not be able to execute searches against “everything” (cross site collection search)

so i am not sure what is this limitation exactly, what is "Search Center and will not be able to execute searches against “everything"? For example let say i as the site admin, create a dash board page using Content Search web parts, where inside the Content Search web-part i will be querying items/documents from multiple site collections, so will the external user be able to view and access those dashboards and their results?

also let say I create a page and i added a Script Editor web part which contain some JavaScript code, to query some lists and libraries using the REST API. so will the external user be able to view the results of those API calls ? which might query cross-site data??