Select is the one exclusive for Amazon, my understanding is that it’s meant to help with counterfeit ones, as only Amazon has them. Very good cards either way.
Not sure how it compares price wise right now, but look at the Amazon basics ones too: they’re made by the same company that bought Lexar and makes the emmc inside the 64gb Deck. Mine has been going strong for about a year now. Speed wise they’re essentially the same: the standard used in the Deck tops out at 104 megabytes per second, so any claims of extra speed are just marketing.
Additionally, to what was already said, the size of storage is giving in Decimal (1000B based) while after formatting it is often shown in Binary (1024B based), which makes the storage look smaller, which it isn't.
And the most of the storage is coming from software stored in your home, not the OS itself. The OS only occupies around 3.3GB on the 5GB root partition:
It should be noted that the way you listed the partitions misses the dual (A/B) install method that the deck uses. There are 2 identical size partitions for root, var, and EFI. When an update occurs. The system installs the new update on the inactive set of partitions and then tells the UEFI to use the other set on the next boot. That doesn't matter too much for 512GB models like your's, but the extra ~5.5GB for the redundant partition layout can be significant for 64GB models.
The df command only shows mounted devices and filesystems. You can use lsblk to show all block devices and their partitions. To format it more nicely to show the labels for each partition, you can use these options: lsblk -o name,mountpoint,partlabel,size.
This is the output from my deck without the microsd card:
Its on the low side but still within reason. Its not just an OS, it's an OS, a full steam install, a web browser( actually 2 of them because steam also packs an embedded browser), a desktop environment, HD animations, HD wallpapers (several of them) and it adds up quick.
Yep. The os isn’t tiny, plus there’s all sorts of extra stuff. For example 1gb is gone on every Deck because that’s reserved for swap… and then shader cache sneaks in and depending on the game you might run out of space in no time.