If I may add to that, the rules defined in a .kopiaignore file are relative to the folder where that file lives. Hence, we can use more than one.
For example, I have ~/.kopiaignore that ignores .Trash/ (who would want to back up garbage?).
I have ~/Library/.kopiaignore that ignores .Cache/ (among other things)
Then I also have ~/Downloads/.kopiaignore that contains
*.app
*.bz2
*.dmg
*.gz
*.tar
*.zip
as those are all downloadable content that I don’t need to back up.
But I don’t want to ignore those file types from everywhere in my $HOME.
Hope that helps.