Can I use it fully offline?
How do I back it up to USB drive?
What does the day-to-day operation of Pass compared to Keepass look like?
I am trying to learn it as I want to use it, as I think that keepass is bloated for my use case, and I would appreciate any help here.
I use qtpass as a GUI for pass
Yes, it is fully offline, you can back it up by any mean you could any other file, and it should be fine as the files are encrypted (should store the keys separated), can be a USB, an external drive, another computer in your LAN, a git repo, nextcloud, syncthing.
You copy and paste the files
As I said I use qtpass as a GUI so, open qtpass, search for the specific password file, double click, put the password for my gpg key and then the password I need is stored in clipboard for 30sec (this is customizable or can be disabled) and I paste it where I need it.
If I need to store a new password, just use the add password button, and input the data, it is that simple.