Hi,
User assigned Managed identities (Managed Identities) are standalone Azure resource and when you check that view it shows only those resources. System assigned managed identities are not standalone resource, they are part of the resource to which are created for so they are not visible under Managed Identities. As managed identities are standalone resource they can exists on your own and can be assigned to multiple resources where the system assigned managed identities cannot live on their own and are deleted with the resource as well. They also cannot be assigned to other resources. You can use the following Azure Resource graph query to see resource that have system assigned identity:
resources | where identity.type contains "SystemAssigned"
Note that resources table does not cover all Azure resources. Some resources have their own table so you will need to check if there are other resources in other tables that can have system assigned identities and you are interested into them. Here is another example for policy assignments with system assigned identities:
policyresources
| where type =~ 'microsoft.authorization/policyassignments'
| where identity contains "systemAssigned"
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