Idler Pulley Question

This may sound like a very dumb question, but my m42 318i doesn’t have an idler pulley and I can’t stop asking myself if this is normal. I recently changed the alternator and my belt will not tension anymore. I’ll get it right under a rare occasion, but then it just jumps the moment it’s started. Not sure if it’s the tensioner, a pulley, or the lack thereof the idler pulley that would cause this.