Yes, in the SharePoint Online, only modern team sites with an Office 365 Group associated can set the site privacy and public in the privacy settings.
In “Private” SharePoint sites, the Owners of the Office 365 group get added as “Site Collection Administrators” and the members of the Office 365 Group get added to the default Members group of the SharePoint site – anyone else can’t access the SharePoint site.
In “Public” sites, the Owners of the Office 365 group get under “Site Collection Administrators” and Members of the Office 365 group get added to the default members group of the site (Similar to “Private”). In addition, the “Everyone except external users” group is added under the members group of the site.
So, you could add Everyone group in the SharePoint site isn't a Teams site to make it public. Or you could manually grant permissions for users you want in the SharePoint site isn't a Teams site.
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