What is the difference between Photoshop CC 2019 and Photoshop 2021?

It is better and compatible with more versions. However, we recommend that those who are familiar with the version use that version. Every time I update, I find it quite annoying to have to find the names of commonly used tools or their locations. Adobe so often, it will reposition, or rename more appropriate. I still remember how crazy I was when I updated to the new version when I was rushing to work on the project "Biệt thự Novawordl Phan Thiết". It slows my progress by 1 day 😞

I sympathize with your confusion regarding Adobe's, at times, bewildering naming conventions.

Back when Creative Suite (CS4, CS5, CS6) was a thing, Adobe introduced the "CC" (or Creative Cloud) designation to differentiate Cloud subscription products from Creative Suite products. CC was used for a decade.

Then, in one of their regular re-branding exercises, they decided to drop CC when Photoshop 2020 was released since Creative Suite versions have long been discontinued and there is now zero risk of confusion with Cloud versions.

So Cloud version naming has gone from:

Photoshop CC 2019 to

Photoshop 2020

Photoshop 2021

"And finally, is Photoshop 2021 also technically Photoshop CC"

In short, yes. All current and future versions of Photoshop are CC versions although CC is now a dated and redundant reference to the Cloud versions.

Beware that older CC tutorials may no longer apply to current Cloud versions where new features have been added or existing features have been changed or deprecated. Where possible, it's always better to follow tutorials which are designed for the same version of Photoshop that you're using (current version is 2021).

That may not always be possible if the author has not updated the tutorial. If the feature has not changed in later versions then updating the tutorial may not be necessary. But always keep your eye out for a current version tutorial. You may find that an entire workflow in an earlier tutorial has been replaced by a brand new, easy-to-use feature in a later version.

No need to keep old versions installed unless you have compelling reasons to do so such as plugins which don't work on latest versions. In your case, I would just keep the latest version installed.

You should be able to uninstall the older version using the Creative Cloud application. You don't need both, I can't see why you'd want an older version around. 

Yes, both are 'technically' Photoshop CC. Just one's a very old version.