This is just going to be a very short form example showing some code to add an Additional Admin to every SharePoint online site.
The Code
The code below will find all modern, teams and publishing site collections, it will then loop through them all and add a defined user as an additional admin.
Be sure to change the *orgName* and *emailOfOwnerToAdd* sections to your own details.
Connect-PnPOnline -Url https://*orgName*-admin.sharepoint.com -Credentials (Get-Credential)
$Sites = Get-PnPTenantSite | where {($_.Template -eq "GROUP#0") -or ($_.Template -eq "TEAMCHANNEL#0") -or ($_.Template -eq "SITEPAGEPUBLISHING#0")}
for($x = 0; $x -lt $Sites.Count; $x++) {
Set-PnPTenantSite -Url $Sites[$x].Url -Owners "*emailOfOwnerToAdd*"
Write-Host "Updating $($Sites[$x].Title), Site $($x)"
}
Sysadmin Central 