You can pretty much use any keyboard you want with USB. I used to use my tablet with one of those usb thinkpad keyboards - thin and easy to carry around.
For my phone - I looked at some folding Bluetooth keyboards on aliexpress which /look/ nice but at ~$30 it's a little steep for something that might be crap and end up never used/trashed. Maybe on Nov 11th i'll bite the bullet if there is a good sale.
This is a major frustration point for me, as I'm working with a keyboard that's missing one physical key and has two others which only intermittently generate the requested character(s). I have ... workarounds, but they're frustrating.
I've written about this at more length than is good for either my sanity or its reputation here:
Upshot: industry standardisation on device sizes, case attachments, and keyboard layouts would be an exceedingly good thing. The pictured keyboard/case (a Logitech device) seems to be among the better general options, though I question its attachment hardware.
Again, given the near-ideal nature of the form-factor, the industry fuckwittedness here is pretty staggering. I've spent more time than I care to think combing through Amazon, Newegg, and other shopping sites (product descriptions themselves make this tedious), and through various online forums and discussions.
The ability to pretty-much instantly, and without concern, switch between keyboarded/landscape and touchscreen/portrait modes is exceptionally useful. "Kickstand" type cases, and loose keyboards, don't offer this, or the ability to use the device in my lap (as I am now) as well as on a table or desk, or (in tablet mode) freestanding.
Much as I don't care for iOS devices, the standardisation and sales volume Apple have attained make for a far superior accessories market than Android. The killer there is the lack of a true compute environment (shell, dev tools) on iOS.
Other options include laptops (including hybrids), though for my use case (many, many, many formatted documents), landscape-mode-only is a tremendous negative, and I am exceedingly averse to all-in-one, hinge-based designs. (For all its faults, Bluetooth doesn't physically wear.)
