Get a list of application and delegated permissions for a service principal, similar to what the Azure Portal shows

Get-MgServicePrincipalPermission.ps1

#requires -version 7 -module Microsoft.Graph.Applications
using namespace Microsoft.Graph.PowerShell.Models
using namespace System.Collections.Generic

enum MicrosoftGraphServicePrincipalType {
  Application
  Delegated
}

class MgServicePrincipalPermission {
  [MicrosoftGraphServicePrincipal]$ServicePrincipal
  [string]$Id
  [MicrosoftGraphServicePrincipalType]$Type
  [string]$User
  [string]$ResourceName
  [string]$Permission
  [string]$PermissionDisplayName
  [string]$Description
  [Nullable[DateTime]]$CreatedDateTime
}

Update-TypeData -TypeName 'MgServicePrincipalPermission' -DefaultDisplayPropertySet 'ServicePrincipal', 'ResourceName', 'User', 'Permission' -Force | Out-Null

function Get-MgServicePrincipalPermission {
  <#
  .SYNOPSIS
  Retrieves the permissions assigned to a service principal, providing a similar output to what is on the Azure Portal Screen.
  .EXAMPLE
  Get-MgServicePrincipal -Search 'displayname:MyServicePrincipal' -Con eventual -Cou count | Get-MgServicePrincipalPermission

  #>
  [CmdletBinding(DefaultParameterSetName = 'Object')]
  param(
    [Parameter(ParameterSetName = 'Id', Position = 0, Mandatory, ValueFromPipelineByPropertyName)]
    [Alias('Id')]
    [string]$ServicePrincipalId,

    [Parameter(ParameterSetName = 'Object', Position = 0, Mandatory, ValueFromPipeline)]
    [Microsoft.Graph.PowerShell.Models.MicrosoftGraphServicePrincipal]$ServicePrincipal
  )
  begin {
    #We use this to cache app info for permission lookups
    [Dictionary[string, MicrosoftGraphServicePrincipal]]$spCache = @{}
  }
  process {
    $ErrorActionPreference = 'Stop'
    $ServicePrincipal ??= Get-MgServicePrincipal -ServicePrincipalId $ServicePrincipalId

    #Add a ToString to the serviceprincipal so it summarizes correctly in the formatting
    $ServicePrincipal | Add-Member -MemberType ScriptMethod -Name ToString -Value { $this.DisplayName } -Force

    #When using Mandatory above, it becomes an empty string instead of null, so null conditional cannot be used.
    if ([string]::IsNullOrEmpty($ServicePrincipalId)) {
      $ServicePrincipalId = $ServicePrincipal.Id
    }

    $appPermissions = Get-MgServicePrincipalAppRoleAssignment -ServicePrincipalId $ServicePrincipalId
    | Where-Object DeletedDateTime -EQ $null


    foreach ($app in $appPermissions) {
      $spCache[$app.ResourceId] ??= Get-MgServicePrincipal -ServicePrincipalId $app.ResourceId

      [MicrosoftGraphAppRole]$role = $spCache[$app.ResourceId].AppRoles
      | Where-Object Id -EQ $app.AppRoleId

      if (-not $Role) { throw "No matching permission found for AppRoleID $($app.AppRoleId). This is a bug" }
      [MgServicePrincipalPermission]@{
        ServicePrincipal      = $ServicePrincipal
        Id                    = $app.Id
        Type                  = 'Application'
        User                  = '[Application]'
        ResourceName          = $app.ResourceDisplayName
        Permission            = $role.Value
        PermissionDisplayName = $role.DisplayName
        Description           = $role.Description
        CreatedDateTime       = $app.CreatedDateTime
      }
    }

    $delegatedPermissions = Get-MgServicePrincipalOauth2PermissionGrant -ServicePrincipalId $ServicePrincipalId

    foreach ($permission in $delegatedPermissions) {
      if (-not $spCache[$permission.ResourceId]) {
        $spCache[$permission.ResourceId] = Get-MgServicePrincipal -ServicePrincipalId $permission.ResourceId
        | Add-Member -MemberType ScriptMethod -Name ToString -Value { $this.DisplayName } -Force -PassThru
      }

      $resource = $spCache[$permission.ResourceId]

      foreach ($scope in $permission.Scope.split(' ')) {
        $role = $resource.AppRoles | Where-Object Value -EQ $scope
        [MgServicePrincipalPermission]@{
          ServicePrincipal      = $ServicePrincipal
          Id                    = $scope.Id
          Type                  = 'Delegated'
          User                  = $permission.ConsentType -eq 'AllPrincipals' ? 'All' : $permission.PrincipalId
          ResourceName          = $resource.DisplayName
          Permission            = $scope
          PermissionDisplayName = $role.DisplayName
          Description           = $role.Description
          CreatedDateTime       = $null
        }
      }
    }
  }
}

I've updated the script to include the original service principal as a reference while still reporting the name, and use a custom type for the permissions for better formatting.

Thanks @JustinGrote :)) Saved me reinventing the wheel here. Modified it a bit to list all the permissions in a gridview so I can select and delete Application permissions.