NewB with DS223j + macbook pro with a few questions

I got sick of paying the annual fees for storage of photos and videos so I bought the Synology DS223j recently with two Seagate IronWolf 12TB. I got it setup, downloaded the files to backup storage from phone.

I'm having issues with my macbook I think:

1. I have two 12TB internal drives in the machine, I assume one will auto copy the files to the other? Or do I need to set this up?

2. When I try to upload files from my hard drive to the NAS, it keeps telling me my computer doesn't have enough storage. I googled how to transfer files from macbook to the NAS and it said to use finder so I am, but apparently that is not correct? I have the assistant but I don't think that is helping either?

3. I want to be able to just drag and drop stuff to the NAS and I'm not understanding how to do this? Bypassing my macbook.

Hope this makes sense.