Hi @Rene Wagner ,
You can use https://tenantname.sharepoint.com/sites/contentTypeHub url to add content type so. Then you add required column to the content type.
This will be avaiable in all other site collections.
sample:
Connect-PnPOnline https://tenantname.sharepoint.com/sites/contentTypeHub
Add-PnPContentType -Name "Pnp-Content" -Description "Created using Pnp-PS" -Group "Pnp-PS" -ParentContentType (Get-PnPContentType -Identity 0x0101)