Pretty much yes. For normal users, the modern almost ubiquitous end-to-end encryption just exists and works, and the two officially use pretty much identical key exchange.

Whatsapp even allows the user to check (offline, or IRL face-to-face, or say by sending the shown check data by a letter) that you and the other end of a given chat are actually an end-to-end encrypted pair (which would bust a third-party man in the middle impostoring as your friend for you and vice versa for your friend). Of course, Whatsapp is closed source app (but then again, did you build your Signal yourself from source on your device, and do you trust the source audits...).

So, probably depends on why you need it. Nice to have, especially as it should reduce Whatsapp to just use your metadata for marketing purposes (like to whom and when and how often) and not the actual message contents. Signal pf course doesn't do even that.

Any source for actual difference in offered protection design? (There are helmets approved for both, given that the actual test requirements for the head protection are nearly identical. To project with the impact from about head-height to the solid ground below. That is, a fall.)

The cooling of the head, or keeping it warm, is the obvious difference in design. Most bike helmets are perforated nearly to oblivion resulting in very lightweight helmets. The helmets that suit both for both are too warm for sportive cycling, work decently for leisurely commuting with an upright bicycle, and excel as a winter helmet for such cycling.

Any sources for the different protection design? (I have a strong impression that both bicycling and skiing helmets are designed and tested for a very similar crash. An impact of the head with the ground, with a velocity corresponding to a fall from about head-height of a tall person. There are also helmets that are rated for both activities, but which only suit leisurely cycling for the reason underlined below. They are, however, excellent for winter cycling at a slow or a moderate pace.)

The main difference is in the need for cooling (a bicycle helmet needs to keep your head cool whilst exercising). The skiing helmet needs to keep your head warm against the windchill of going fast in the colder winter weather.

I have also a very strong impression that no bicycle helmet is designed in particular for an impact with a motor vehicle. The required test is to protect against the head hitting the solid ground from the height of an person cycling.

First, we can note that both types helmets have essentially identical requirements for attenuating impacts (and nearly identical tests to validate their performance on crashes). So, there is no crash-safety reason for the difference. (Like, a bike helmet is not particularly designed to protect against an impact with a car.)

Thus, the main reason is due to different needs for ability to cool of ones head. A person driving a bicycle needs much better cooling (two reasons: typically warmer weather and the act of cycling itself). Thus, a bike helmet has plenty of cooling holes. The skiing helmet does not need these, thus, one can use a shell with far less ventilation (resulting in a helmet that is a bit more robust against wear and tear, in addition to a nice smoother look of helmet).

There are, however, helmets that can be used for both; and which are sold for both. These are clearly in between the fully smooth skiing helmet and the highly perforated bicycling helmet. A few holes, sometimes with adjustments. (Though, given the limited maximum ventilation, such helmets only suit leisurely cycling. A such, they are also excellent for winter cycling, especially if done at leisurely pace, say, to commute at a pace where one does not need to shower or even change clothes at destination.)

No, because I read the instructions. ;-) (To explain it with similar complexity as in the question, not to a literal 5-year old. Also, as I expected there to be plenty of literal ELI5 answers.)

The sound you hear is a wave that is a sum of all sounds around you. Waves have a few relevant properties: They travel at a known velocity, and they are additive.

To cancel such wave in your ear: we measure the wave just outside the ear and play its inverse with a small delay from the earphone. Notably, this only cancels the sound in a very small region around the inner side of the earphone. Everywhere else it adds its miniscule amount of more sound to the wave.

For best results: You need a good microphone in both earphones, and a good algorithm to slightly alter the wave, to mimic hiw it will be altered by the earlobe (as the in-ear earphone sound is not altered by the earlobe identically to the sound coming from the outside). Fortunately, we can tune this individually: place a second microphone inside each ear canal (near the very tip of the earphone), and measure which delay and which amplitude modifications reduce the sound the most.

A good analogue: Look at the waves in the see. Measure the height of the wave. If it is above the mean water level, push the water down with a paddle you have placed under the surface. If it is below, push the water up. If you move your paddle at just the right speed for a given measurement, you can destroy the wave around your paddle (whilst creating a new wave around your paddle, propagating outwards and adding a bit to the waves everywhere else in the sea).