OK, a terminology thing:
The "141 walkups" are not hotels, they are buildings where several floors have been converted into one-woman facilities. The lower floors will be shops and there may be regular residences or cheap hotels on some of the other floors.
Largest ones are:
Fuji Building (Causeway Bay/Wanchai) on HK Island.
James S Lee Building (Tsim Sha Tsui - bottom of the Kowloon peninsula)
King Hing Building (Mongkok - middle of the Kowloon peninsula).
So you'll see people talk about Fuji, JSL, and KH.
If you want some idea of what these buildings are like here are two videos by the same person, I think.
The first video is shot in Phoenix Building (Causeway Bay on HK Island, east of the central district with the banks and stuff). Only two floors are girls, so smaller than the above.
At around 12 seconds the G5 door is Sammi, one of the legendary milfs there.
Her current advert name is "Happy":
The second video is in Sheung Wan, west of the central district. In this cases he got some girls to actually open the doors.
It's not super cheap, but almost always includes showering together in a miniscule shower (they want you clean) and very energetic service. They want the turn over.
Last time I was in HK, a year ago, I visited: Sammi/Happy in Phoenix 600HKD (130NZD) for about 30-45 mins; A HK girl in James Lee that would also have been 600 but I elected to have a 30 min massage first, so it was 900 all up; A Thai girl in James Lee. Quoted 500 and was 800 including a massage.
There tend to be signs and sometimes photos (for longer-term workers like Sammi) on the doors. If they are busy there will be a sign. Very occasionally there will be a "no foreigners" sign,, but that's never been a problem for me. If you keep an eye on the Forum you may get some intel on someone really good.
There's no obligation to go in if you knock, so you can browse. Obviously use your head and pick someone who has a friendly vibe. The good thing is that you're interacting directly with the provider - there's no middle man/woman
There are security cameras everywhere and the girls have several different views of the corridor on their screens. It's pretty safe unless you behave like an idiot - noone wants trouble that will ruin their business.
If you want to try "Hotel walk-in" you obviously can't just walk in, you first have to contact an agent, typically on Tele gram or Whats App or SMS and they direct you to a particular hotel. I've never tried that. It looks like it tends to be more expensive and better quality but the downside is that it's like punting in NZ - you spend time getting oranised and getting there so it's annoying to walk away. With 141 walkup the next door is only 2 metres away...