You can get them via the Graph API's approvalStage property: https://learn.microsoft.com/en-us/graph/api/resources/approvalstage?view=graph-rest-beta
For example, this query returns the policy associated with the Attribute Definition Reader role:
GET https://graph.microsoft.com/beta/policies/roleManagementPolicyAssignments?$filter=scopeId eq '/' and scopeType eq 'Directory' and roleDefinitionId eq '1d336d2c-4ae8-42ef-9711-b3604ce3fc2c'
Once you have the policyID, get the rules and in particular, the Approval requirements:
GET https://graph.microsoft.com/beta/policies/roleManagementPolicies/Directory_923712ba-352a-4eda-bece-09d0684d0cfb_54587f2b-c596-41bc-a576-149c3430d49a/rules/Approval_EndUser_Assignment
Therein, under the Settings > ApprovalStage > primaryApprovers you will find the list of users.
Additional details here: https://learn.microsoft.com/en-us/graph/api/resources/privilegedidentitymanagementv3-overview?view=graph-rest-beta#role-settings-and-pim