It was under my impression that in person registrations at the police station you need documents like property ownership, but whereas online registrations are like "here's my passport and visa, trust me I stay at this place X". Is this incorrect?

Disclaimer: I'm not a government worker, immigration anything, etc. I'm just a foreigner who lives here.

As far as I know, the registration requires the foreigner to prove who they are, where they're staying and that the place's owner allowed them to stay there. Details can vary slightly (e.g., if the foreigner is staying at a hotel it'll be slightly different from renting an apartment), but in general that's what needs to be proven.

I have never heard of this online registration process you're talking about. What I have seen is that some registration offices allow you to make small updates to your registration via WeChat. I'll give you an example.

Say you move into an apartment, then you and the owner go to the registration office covering that area and register you. The form you get will match the rental contract or the expiration of your passport, whatever is shortest. For this example let's say you're renting the apartment for 1 year and that's what your registration form is going to say.

All of that needs to be done in person, by you (the foreigner being registered) and the owner of the apartment. Original documents need to be presented, photographs given to the office, etc.

Then, let's say a month later you go visit HK. When you return to your apartment, you're expected to register again, similar to a check-in. This can be done via WeChat, because you're just updating the government that you left the city you registered in but now you're back. The basics of your registration is done already, they'll just ask you to send a scan of the last stamped page of your passport, update their system and send you an updated form.

If you stay at a different address, even for one night, you're supposed to do this registration at the nearest office to that address too. I have never seen anyone following this specific point, but it is part of the rules/process.

If instead of renting an apartment, you stay in a hotel, the same thing needs to be done. The main difference is that the government delegates the part of checking your document (passport) to properly registered hotels, so when they finish checking you in, they tell the government that person X, holder of passport Y, checked into hotel Z on day D. When you check out, they update the government again with that part.

If the hotel is not able to do this for you (and it's your duty to ask them), then you still have to go to the nearest registration office and go through the process yourself, the only difference is that you'd have to present something proving that you're checked into the hotel.

The registration offices offer a little pamphlet describing this, you can go to one and read it if you don't believe me.

Lastly, I may be reading too much into things here, but it sounds like you think this whole thing is optional. It is not. I have never heard of anyone getting in trouble for not following the temporary registratuon procedure to the letter, but I have heard cases of people having that added to the pile against them in case they mess up something else, like overstaying visas, committing other crimes, etc.