Based on your description, I understand that you want to share a whole SharePoint with external users. Per my knowledge, you only could share sites individually with external users. Here're steps:
1.Enable external sharing in the tenant level. Go to SharePoint admin center -> Polices -> Sharing -> Set Anyone.
2.Enable external sharing in the site level. Go to SharePoint admin center -> Sites -> Active sites -> Select the site you want to share -> Sharing -> Set Anyone.
3.Share SharePoint sites with external users. Go to Site settings -> Site permissions -> Share site only.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.