SharePoint Online: How to Create a Site Design from Existing Site using PowerShell?

How to Create a New Site Collection from an Existing site in SharePoint Online?
With Get-SPOSiteScriptFromWeb cmdlet, we can create a site design from an existing site collection and then use the site design to create new instances of the site. The cmdlet is a new feature addition to SharePoint Online! It extracts the site schema from a given site and can be used to create copies of any existing site structure. Here is the basic syntax and example:
Syntax:
Get-SPOSiteScriptFromWeb -WebUrl $siteUrl -IncludeTheming -IncludeBranding -IncludeSiteExternalSharingCapability -IncludeRegionalSettings -IncludeLists $RelativeListUrls

Example:
Get-SPOSiteScriptFromWeb -WebURL https://crescent.sharepoint.com/sites/marketing  -IncludeBranding -IncludeTheme -IncludeRegionalSettings -IncludeSiteExternalSharingCapability -IncludedLists ("Lists/work","Lists/Marketing%20Documents")

Create Site Design from an Existing site using PowerShell
Before using this cmdlet, make sure you have the updated SharePoint Online PowerShell module installed How to Update SharePoint Online PowerShell Module?!

#Define Parameters
$AdminCenterURL = "https://crescent-admin.sharepoint.com"
$SiteURL = "https://crescent.sharepoint.com"
 
#Connect to SharePoint Online
Connect-SPOService -Url $AdminCenterURL -credential (Get-Credential)
 
#Get the site schema
$SiteSchema = Get-SPOSiteScriptFromWeb -WebURL $SiteURL -IncludeBranding -IncludeTheme -IncludeRegionalSettings -IncludeSiteExternalSharingCapability -IncludedLists ("Lists/Events","Shared%20Documents")
 
#Add site schema as Site Script 
$SiteScript = Add-SPOSiteScript -Title "Crescent Site Template v1" -Content $SiteSchema
 
#Create a Site Design for Team Site template
$SiteDesign = Add-SPOSiteDesign -Title "Crescent Site Template v1" -WebTemplate 64 -SiteScripts $SiteScript.Id

Once the site template is created, You can create a new site collection based on the site design you had created:

or apply the site design on existing sites.