Back up before you do this and read why it won’t do what you want first.
Here’s how to do what you’re asking: chown -R user /
You’ll need to be root or use sudo to do that.
Here’s why it won’t do what you want: the arr programs each have their own user and group and they expect to be able to freely modify files they are allowed to through the permission system and to be prevented from doing so for files they’re not supposed to touch through that same system. It doesn’t matter if your user has ownership of all files on the system because your username isn’t the one those programs will be operating under when they go to move something from your incoming directory to your storage drive.
You could change them to all use the same username, yours, by editing their config files, but that’s not recommended and the programs are not tested in that configuration or designed to be run that way.
Not only will operating a single user system not fix the problem you’re having, making your username the single user isn’t the right way to do it. Running in single user mode is done as the root user and it has a special run level, 1. There are five other run levels above it (not counting zero, which is when the computer is not running).
Tldr: don’t do what I just told you how to, it won’t fix your problems. Learn the permissions system and fix your permissions problems instead or use a platform for the arrs that you’re more comfortable with.