Time Machine

Just had some questions regarding TIME MACHINE, I know that it backs up everything but I dont fully understand how it works.

so If I backup up my computer to an ext.HD through TIME MACHINE, is it pointless to copy files from my computer to an ext.HD since its already backed up?

Am I just making duplicates and eating up space this way?

Also, does Time Machine back up the files I copied into the int.HD? I'm thinking I have quadruplet copies of everything in there. 😕

also...I know that Time Machine starts deleting old backups, but I dont know how old a file has to be before this happens

and I see that it notifies you after the deletion, but why not before?

How do I find out how much memory TIme Machine is using up?

I have the same confusion with anything iCLOUD.😐