I want to capture the Territory Ids the user is assigned to. I wrote the trigger and the logs show the value that i am expecting, but the record is not getting updated. Could you suggest me please?

trigger preventTerritoryAssignmentDeletion on User (before update) {
    
    Set<Id> userIds = new Set<Id>();
    List<User> UT2List = new List<User>();
   
    for(User u : Trigger.New){
        if(u.Resource_ID__c == 'testresourceid'){
            //u.Territory_Backup_IDs__c = 'workinig';
            userIds.add(u.Id);
        }
    }
    
        UT2List = [SELECT ID,Territory_Backup_IDs__c, (SELECT ID,Territory2Id FROM UserTerritory2Associations) FROM User WHERE Id IN :userIds];
		
        for(User u : UT2List){
            string terrString =',';
            for(UserTerritory2Association ut2a : u.UserTerritory2Associations){
                terrString = terrString + ut2a.Territory2Id +',';
                system.debug('terrString inside inner for loop'+terrString);
               
            }
           
            system.debug('terrString in out for loop'+terrString);
            system.debug('u.Territory_Backup_IDs__c'+ u.Territory_Backup_IDs__c);
            u.Territory_Backup_IDs__c =u.Territory_Backup_IDs__c+ terrString;
            system.debug('u.Territory_Backup_IDs__c'+ u.Territory_Backup_IDs__c);
            }
          
    
}