One question about the "backup" ....mystery to me

hey all,

i dont fully understand how the backup is meant to be and before experimenting i prefer to ask. So, i understand how it works and all is backed up fine on my google drive. Now, i have all those logs/pics/videos that are backed up fine also locally saved on the phone via whatsapp.
I want to delete the locally saved stuff but keep it in the cloud.

But when i delete it locally and whatsapp does another backup - does it backup then only whats locally available and overwrites/deletes the files that were backed up before?

Can i just delete the local whatsapp stuff, after back up and it will automatically grab whatever is needed in the moment from the cloud, from the back up?

In other words, will it keep adding to the already stuff already in the backup or create a new backup everytime based on whats locally stored in that moment connected to whatsapp?

thanks,
ducky