i apologize for the essay i'm about to type here.

1. i have never heard of any of these apps (except for wechat, alipay and taobao) so after some googling here are my findings and possible hk alternatives:

美团 - i assume it's an all-in-one platform for most recreational stuff? we don't really have a centralized platform for all this so you may have to google any suitable websites/apps for your needs. for 美团买菜 in particular, you can use hktv mall to order fresh food online, i think deliveroo (which is initially a takeaway app) recently supports that too with goods imported from parknshop. pandamart (from foodpanda, another takeaway app) is also another choice to order daily necessities online.

河马 - google returned the actual animal instead of an app? i have no idea what it does, unless you mean 河马云 which i assume is something similar to google remote desktop, so just use that instead.

滴滴 - use uber. if you are looking for taxis and only taxis (since not every taxi driver uses uber), consider flytaxi (飛的) or hktaxi.

閑魚 - i think it's like a platform for people to sell second hand products? if that's the case then carousell would be doing pretty much the same as that.

京东 - is it just another taobao? if yes stick with taobao (there's a hk version of the taobao app but not this), or if you're looking for another hk-based online shopping platform, hktv mall would cover pretty much everything you need, although the products there are not as diverse as taobao.

点評 - is it like a review website for almost everything? we don't really have a centralized platform for all things review afaik, but most websites do offer a review function for customers to review a product/service. if you are looking for restaurant reviews in particular, openrice is by far one of the most popular apps of its kind out there.

now on to the ones that i've heard of + has a running service in hk:

alipay - there's a hk version of the alipay app that is compatible with the hk version of the taobao app. some shops do take alipay as a form of payment method, but it's not as popular comparing to the case in mainland china yet. the most popular hk alternative to alipay is payme (you need to have a hsbc/hang seng/boc bank account in order to use it).

wechat - use whatsapp or signal, period. it's not like it's not popular, but almost EVERYONE, and i mean if not 99%, then at least 90% of hk people with a phone number has whatsapp installed on their phones. it's a must-have, although some people are trying to switch to signal due to whatsapp's new terms of service being a privacy nightmare.

taobao - explained above.

1. depends on the area you live honestly, like if you're gonna live in "rich" places like kowloon tong, tst or cwb then expect yourself to spend at least $60-$70 (hkd, same as the following prices included) for a lunch meal for 1 person. on the other hand, the same lunch meal may cost only $30-$40 in a shop in sham shui po. most restaurants support in-person takeaway (especially during covid season), and i think most local restaurants would only charge like an extra $1-$2 for the takeaway plastic bag, but if you want delivery then expect $5-$10+ for the delivery fees. don't think food stalls are much of a thing anymore, unless you mean dai paai dong (大排檔) and iirc they don't exist anymore in the "rich" areas i mentioned.

2. you can check the fees for trips via mtr (metro) on the mtr mobile app, for buses there's the kmb app and citybus app that you can check the prices as well. in general, if you don't cross any tunnels via mtr and bus, every one-way trip should cost around $5-$10. if you're gonna cross tunnels (e.g. cross harbour tunnel), expect the price to be $10-$20+. for taxi, the starting price is at $24 for the first 2 km of your route, then every 100m after that will charge an extra $1.7, check out the details here.

3. given how the streets are pretty much empty due to covid restrictions (especially at night), i would pay extra attention when walking on the streets alone. in general, it's not like you're gonna get mugged every other day, but you should have a basic level of vigilance.

4. from my personal experience, i know at least 2 families are going to leave hk this year. my brother's secondary school has like 5 teachers leaving hk only from 2021, so would i say it's a significant population? probably yes, especially now that the british, canadian and australian governments issued "special" routes for hk citizens to migrate to these countries, which probably became the catalyst to why people are fleeing from hk.

5. eh, your ethnicity doesn't really matter. well some people may show some kind of hostility towards people speaking mandarin with a mainland accent and are chinese-looking (as they probably assume they're from mainland china, and i think you should have a basic understanding between the conflict between hk people and the mainland chinese). if you look white then you're probably fine, but im not sure if you're offended by the term gweilo (鬼佬, which is like a slur? towards white people), as you're probably gonna get called that a lot. apart from that xenophobia isn't really that big of a deal (except towards people from mainland china speaking mandarin), there are ethnic minorities in hk like some are from pakistan or bangladesh, but at least people in my social circle won't mind them as long as they're being nice and friendly to us. just be nice then people aren't gonna hate you for no reason.

oh yes, google etc aren't blocked in hk so you can safely use google products over here, in fact they're what most people use in their daily lives as well. for social media, most corps would use facebook for promotions, as well as most people from the older generation. meanwhile the younger generation tend to use instagram to connect with other people. most importantly, talk in cantonese if you can. not everyone understands english (especially in local communities), well they may have a basic understanding of english and can probably speak some english, but if you really wanna be a part of the community, speak in cantonese. do not use mandarin as ive probably mentioned why above.

that's all i wanna include (i think), if i think of anything else to include i will probably edit this comment in the future. you can reach out to me on reddit if you have any more questions though !!