how to join two lists?

private async Task<AccessGroupDataSet> PrepareDataSet()
{
   var accessGroups = await GetAccessGroups();

   var territories = ( await GetTerritories( accessGroups ) );

   var groupedTerritories = territories
      .SelectMany( territory => territory.ParentGroups
          .Select( parentGroup => (
              Id: parentGroup.Id,
              Title: parentGroup.Title,
              TerritoryTitle: territory.Title,
              TerritoryDescription: territory.Description,
              TerritoryRules: territory.Rules,
              ScheduleId: accessGroups
                  .SelectMany( accessGroup => accessGroup.Compositions )
                     .Where( composition => composition.AccessGroupId == parentGroup.Id )
                     .Select( composition => composition.AccessScheduleId )
                     .FirstOrDefault()) ) )
       .GroupBy( territory => _parameters.CanGroupBySchedule
            ? territory.ScheduleId
            : territory.Id );
    }

private async Task<AccessGroupDataSet> PrepareDataSet()
{
   var accessGroups = await GetAccessGroups();

   var territories = ( await GetTerritories( accessGroups ) );

   var groupedTerritories = territories
      .SelectMany( territory => territory.ParentGroups
          .Select( parentGroup => (
              Id: parentGroup.Id,
              Title: parentGroup.Title,
              TerritoryTitle: territory.Title,
              TerritoryDescription: territory.Description,
              TerritoryRules: territory.Rules,
              ScheduleId: accessGroups
                  .SelectMany( accessGroup => accessGroup.Compositions )
                     .Where( composition => composition.AccessGroupId == parentGroup.Id )
                     .Select( composition => composition.AccessScheduleId )
                     .FirstOrDefault()) ) )
       .GroupBy( territory => _parameters.CanGroupBySchedule
            ? territory.ScheduleId
            : territory.Id );
    }