It’s a pretty new thing. In recent years many new TLDs have been added.
See here for a full list currently supported on the open web: https://data.iana.org/TLD/tlds-alpha-by-domain.txt
Companies are now allowed to add TLDs to this, if they can provide a reason and the infrastructure needed to make them usable.
Amazon owning those TLDs and not allowing registration for it is not okay in my opinion. The words “book” and “read” are not exclusively used by Amazon.
Here is some insight into the process: https://www.icann.org/en/topics/new-gtlds/draft-rfp-clean-12nov10-en.pdf
Can you choose 4K from the little gear icon in the bottom right corner of the player?
Not trying to defend YouTube, but I think they align video quality to network speed so you can watch without buffering.