If you want to share a site with other tenant, the only way is to enable external sharing.
You should enable external sharing in both tenant and site level.
1.Enable external sharing in the tenant level. Go to SharePoint admin center -> Polices -> Sharing -> Allow new and existing guests.
2.Enable external sharing in the tenant site level. Go to SharePoint admin center -> Sites -> Active sites -> Select the site -> Sharing -> Allow new and existing guests.
3.Then go to the Site permissions -> Share the site.
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