How to remove all sites bar and change the domain name in SharePoint modern site

bhavaM  I beleive from March 31, 2022 no longer we can hide app bar . You can find more details over here.
https://learn.microsoft.com/en-us/viva/connections/sharepoint-app-bar

In order to change domain you have to use power shell. Before you change you have to check the domain is aviable or not via admin center or Azure AD portal. If aviable please reserve it and then use powershell to update domain.

Add new domain : https://aka.ms/SPORenameAddDomain

Update domain in sharepoint:

Connect tenant : Connect-SPOService -Url "https://yourtenant-admin.sharepoint.com"

Update : Start-SPOTenantRename -DomainName <DomainName> -ScheduledDateTime <YYYY-MM-DDTHH:MM:SS>

Article : https://learn.microsoft.com/en-us/sharepoint/change-your-sharepoint-domain-name

Regards
Koti