I definitely hope we never find out as it would mean WW3 and a likely nuclear holocaust. I do expect that stealth air craft detection is possible, especially with modern technology where you can integrate multispectrum data. Radar is just one way to look for the aircraft, it produces a whole spectrum of emissions, such as heat and sound. These are pretty much impossible to hide, and I imagine that once you see an anomaly in one spectrum it allows to check more closely across different spectrums to home in on the target.

In terms of subs, Russia and US are about even in terms of the number of subs, while China has the most at the moment. Russian and Yasen class subs can carry more weapons, and in particular hypersonics which US doesn't have. I imagine there's been a lot of tech transfer happening between Russia and China here as well which is what allowed China to develop their own fleet so rapidly. Also agree that the most likely scenario is that US sub got intercepted as opposed to having run into the ocean floor all on its own.