Back up the operating system as in a restore disk, or backup the whole drive as in a disk image?

Normally computers come with recovery disks etc. This because it didn't have a CDROM drive what am doing to backup this computer now? Usb though the windows recovery in windows?

There's a recovery partition built into the OS boot drive now.  

If you want bootable win 10 install media, you can download it for free here and install on a USB flash drive.  Your windows 10 is digitally activated so no need to manually input a key. 

https://www.microsoft.com/en-us/software-download/windows10

So I understand this, if my desktop goes hay wire for some reason and can boot into windows extra there's a partition on the drive that will allow me to recover? No need to create recovery USBs etc to get back to normal right?

I would always create a recovery USB, simply in case the partitions somehow get damaged or something happens that prevents the recovery partition from being activated.

If you want to create a recovery USB to restore your machine back to it's original factory state, you can use this option: DELL OS Recovery Tool 

If you want to create a recovery USB with just windows on it, you can use the Microsoft media creation tool as explained above.

With my R11 being liquid cooled do I need to check or add any liquid to the machine at all?

No, these units are called AIO (All in ones), and they require no maintenance.

The lifespan is typical several years. I have an H50 AIO that after 8 or so years is still running like a champ.