https://youtu.be/nCwyBPuYK6g?t=167

From my understanding, the buttons have a capacitive touch layer on them.
So there is extra hw required.

So it's just a normal keyboard with something extra implemented in software?

I suppose those horizontal bars might matter and do work like a touchpad.
It would be hard, expensive to implement such on every one key.

Thanks, I get it now. Never seen one in real life before!

If that was the case, wouldn't the smooth circular movements, in the video, i linked above, be impossible to realise, considering these big gaps between the horizontal bars?

Hard to say.
On the other hand, paying attention to the fact that some broken displays even with falling out pieces of glass do work it might be similar capacitive technology as in the screens. So it might use not only those bars.
Either way, thanks for the note.

Good, as this seems to be the only viable choice today, especially regarding hardware support (device drivers) in upstream Linux (= kernel).

For the choice of operating system(s) there are basically only two open sourced "survivors":

1. "Android"
- Google's Android (i.e. "stock Android" *from* Google) is out of reach for a project of this size, IMO. BTW, is this what you called "GMS Android"? And it contains a lot of proprietary components, binding its users to Google services.
- AOSP (Android open source project) is Google's code drop of their Android releases with all proprietary components stripped and some of them substituted by old (open source) ones. It does not deliver, what a regular user expects.
- LineageOS is based on AOSP and adds substitutes for some of the missing / outdated components in order to suit regular users (and they are doing that well).
- BTW, AlienDalvik (by MyriadGroup), which can be licensed along SailfishOS, is also based on (older releases of) AOSP.

2. Nemo mobile / Mer / SailfishOS
- Nemo mobile is based on Mer ("Meego reconstructed"), which is the successor of Meego, which in turn is the successor of Maemo (well, simplified; for details, see Wikipedia). It also is not suitable for regular users, e.g. lacking a finished mobile GUI.
- SailfishOS is also based on Mer and provides (in contrast to Nemo) a proprietary, proper "mobile" GUI, plus the optional integration of AlienDalvik.

All other options are dead (Ubuntu Touch, MozillaOS, WebOS etc.) or dying (e.g. Tizen).

As most people want to run Android apps, but also many people want an alternative to Android, there are really only two options:
a. Dual-Boot with LineageOS and SailfishOS (the latter maybe optionally due to license costs)
b. SailfishOS with AlienDalvik ("single boot")
... and the combination of both:
c. Dual-Boot with LineageOS and SailfishOS with AlienDalvik
*Edit:* Other people added,
d. Preinstalled with LineageOS, which is replaceable by a SailfishOS image ("single boot"), with (d1.) or without (d2.) AlienDalvik.

Note, that a properly working dual-boot setup can be a challenge of its own (technically *and* WRT usability), thus primarily perusing b. or d[1|2]. makes most sense, IMO.

Great summary, although I would conclude with d. LineageOS by default, with an easy way to install Sailfish. Dualboot out of the box would be cool, but is there any well known precedent there?

Problem with Dualboot is that, even though Multirom exists and works well, IIRC it's not available for Android >6.0 and thus LineageOS.

I'd go for LineageOS & easy to install Sailfish.

lol. I meant that old BB nub thing. I hated that thing something fierce. Felt like you were pushing around an old eraser head.