How can I find out where a package is referenced in Architect?

There are package references shown in every package in Architect.
But how can I find out all packages where the single package is used?

You can use this script to display a list of packages that reference your package (you need to run it on your model database):

declare @packageId uniqueidentifier
set @packageId = 'enter_package_id'

select p.Name, p.SchemaExtensionGuid from PackageReference pr
inner join SchemaExtension p on p.SchemaExtensionGuid = pr.refPackageId
where pr.refReferencedPackageId = @packageId