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XTL
There aren’t, really. There are a few antiques and half baked things.
A big problem is that these days, unless you’re the size of Apple or Samsung, it’s impossible to get a reasonable hardware soc and modem other than one which only runs a soon obsolete blob laden android which is going to be EOL before you’ve even finished your design.
The hardware is not there. The firmware/hw data/platform isn’t there even to begin OS work with. And there’s a global shipping, regulation and mobile operator hell waiting on the other side. And a product lifecycle that’s only a few years long.
Yes, I’ve worked for phone manufacturers.
https://ruralfencing.com/products/pocket-size-axe-sharpening-stone
Looks like an axe stone. Can’t recommend a good tutorial other than a scout who knows. But you might find some. The key is a grip that keeps all your fingers on the rim and away from the side you’re using.
It’s the stupid name.