Anniversary Edition is NOT a free upgrade from Special Edition (SE), the version currently available on console.
There IS a free upgrade from SE to Next Gen SE, which does include three pieces of creation club content for ‘free’, but if you want to upgrade to Anniversary Edition with all the other creation club content you still have to pay.
I think it’s the basically a slightly adapted version of the BeoPlay HX’s.
Price here;
Bang & Olufsen Beoplay HX – Comfortable Wireless ANC Over-Ear Headphones, Black Anthracite https://www.amazon.co.uk/dp/B08TZRP19J/ref=cm_sw_r_cp_api_glc_fabc_F11ZY5FCGNWV90A6E591
1 - It depends. You'd be surprised at the potential that's packed into that tiny little guy. The CPU has very strong single threaded performance, I find it matches X1X when overclocked. The GPU feature set is also the best you could ask for in a non-ray tracing device, fully FL 12_1 compliant. OTOH, absolute performance/processing power is limited, and RAM capacity/bandwidth is less than ideal (latency is strong however as it uses LPDDR4-3200). So I guess for making a "Switch-sized" game it's a great system, but you might have trouble with porting games made for more powerful devices.
2 - Yep, DX12 Ultimate ensures a FL 12_2. So XSS/XSX, Turing, Ampere, RDNA2 and Xe HPG will all run the engine no problem! Switch I'm cutting all the 12_2 stuff as it's done in NVN (Nvidia Vulkan for Nintendo)
3 - I got this book for my birthday and it's been just as useful a resource as Microsoft's documentation. The only pre-reqs are solid C++ knowledge. Even if you don't know DX11 or any gfx API it will teach you from the ground up (and if you do know gfx APIs then you'll learn even quicker)