External sharing allowed to only designated domains

To achieve external sharing for specific domains you can follow the below steps:

1. Go to https://admin.onedrive.com/?v=SharingSettings
2. Under Limit external sharing by domain > click on Allow or block sharing with people on selected domains.
3. Click on Add domains
4. Choose any one

Allow only these domains
OR
Block these domains

5. Enter each domain on a new line.
gmail.com
yahoo.com
hotmail.com

Please refer the attached screen shots for your reference.

Reference article:
https://techcommunity.microsoft.com/t5/OneDrive-for-Business/Announcement-New-OneDrive-Admin-Center-Preview/td-p/35918

How to Lock Site Collection in SharePoint Online and have redirect users to another site

Requirement:

When users browse to the site collection https://domain.sharepoint.com/sites/sitecollection1 it should be locked and they should be re-directed to another page and should be notified about the same.

I created a page called it "Lock-Site.aspx" under root site collection https://domain.sharepoint.com/ and gave view permission to everyone group specifically on the page where I have added text message "The site that you are trying to access has been locked by the IT Department. If you have any questions please contact Administrator admin@domain.onmicrosoft.com"

I ran below command to achieve this.

1. Set-SPOSite -Identity https://domain.sharepoint.com/sites/sitecollection1 -LockState "NoAccess"

2. Set-SPOTenant -NoAccessRedirectUrl "https://domain.sharepoint.com/SitePages/Lock-Site.aspx"

Behavior:

When the users try browsing the site https://domain.sharepoint.com/sites/sitecollection1 they would redirect to https://domain.sharepoint.com/SitePages/Lock-Site.aspx which states "The site that you are trying to access has been locked by the IT Department. If you have any questions please contact Administrator admin@domain.onmicrosoft.com"