Whew - where to start. Overall for the mobile app ecosystem and gaming in particular, China is one of the largest markets globally and one that cannot and should not be ignored in most cases. The flip side of the coin is that it is probably the most complex market at scale to navigate from various perspectives (culturalization, regulatory landscape, dominance of local incumbents, fragmented Android ecosystem, etc.) and therefore it is easy to "spin your wheels" wasting significant time and money trying to enter China.
Overall the key thing you should keep in mind is ROI/profitability, not just from a marketing perspective, but your overall investment in trying to penetrate the Chinese market, as there are a lot of actions/steps you can take which may boost your revenue, but may have a large out of pocket and/or opportunity cost such that your efforts may actually be unprofitable and that taking some more simple/less time/$$ consuming steps may actually be profitable.
Game or not?
The first key point in the decision tree is whether your app is a game or not, which should be pretty straightforward. If your app is not a game, then count your lucky blessings. If your app is a game, technically to be released in Mainland China it needs to go through the regulatory review process run by the government which allows you to secure a license code for your game. An online search will go into great detail about the specifics of this process, but it is a pain in the ass and takes many, many months, but the bottom line is that unless you are a wholly owned Chinese company you cannot even submit your game for the review process. So as a game, to be legit, you need to work with a local partner to secure the license code, but nowadays the process is so slow and time consuming, that local mobile game publishers have been forced to become even more particular in what games they partner with.
So what now if I am a game (or not)? iOS vs. Android
So the good news is that currently even though technically every game available in China needs to have a license code, the App Store is still a grey channel for app distribution in that even though there is a field to enter your license code as it relates to China for app submission, there is minimal to no enforcement from the App Store. Sometimes your app or app update will get rejected without the license code, but a simple resubmission usually works. It is possible that this loophole could close at any moment for China on iOS, which would be catatrosphic to the mobile gaming ecosystem.
For non-gaming apps on iOS, it is business as usual (as it relates to distribution in any other country).
On Android, hopefully everyone already knows that Google Play does not exist (officially) in Mainland China and the Android ecosytem is made up of various Android app stores, which luckily have become less fragmented in the past 2-3 years. For games, without the license code (which 99.9% of you reading this will not have), you are out of luck, as all the key Android channels require the license code information so that they do not run afoul of the government and face a beatdown of potentially epic proportins, so you are forced to go the publisher route. For non-games, there are companies out there that will provide Android distribution as a service, helping you with the operational and tech requirements of getting into all the relevant Android channels (protip: don't underestimate the complexity of generating binaries for 10-20 different Android channels).
If you are going to legitimately support the China market, you must translate your app into Chinese (Simplified; Traditional Chinese for Taiwan and Hong Kong markets). I would also recommend translating your app store assets and general presence as well, but don't just translate your app store presence if the app is not also translated, as no one that downloads the app will be able to understand what to do. Going back to my ROI comment early on, if your app or game (especially if your game has narrative that is much harder to translate well) has a lot of text, then do some diligence around what the market opportunity is for your app or game in China first before investing a lot of money to translate it (and also do QA on the translation), as the user tastes in China are unique and the interest in your app may be very low even if it performs well in the West.
Performance focused growth strategy
My general feedback here is that while Facebook and Google as UA channels are non-starters (since they are both blocked in the market), 90% of you should be focused on leveraging the broad reach of many of the Western-HQed UA channels that you currently use for iOS right now, as you are familiar with their teams and tech, and all have large inventory in China on iOS right now. Only until you truly saturate all Western-HQed UA do I recommend exploring local channels, which do exist, but have decent overhead to support since their account team may not speak English, their dashboards and documentation are likely not in English and their platforms are also likely not self-serve. There are also agencies that can act as middlemen for these local UA channels for iOS, but again think about ROI here - it is efficient to max out the Western-HQed UA channels for iOS traffic in China before diving down the rabbit hole of local partners, which will take up a lot of time.
For Android, again for games you are DOA and therefore don't need to worry about Android marketing, but for non-game apps, I would tread very cautiously around UA based marketing for Android, as the ecosystem is still in its infancy and fraud is a huge issue. I'd say just focus on getting into the key Android channels and then explore any visibility you can get within the store.
By doing all the above, I think you can capture meaningful incremental revenue from China for a product you have already built without a massive amount of effort and complexity, even if you just focus on iOS, as even a small share of the massive app revenue pie, including in-app advertising revenue, is going to be quite meaningful for most based on China's scale. That being said, be realistic about your app's potential in China, as like most markets in Asia, the charts are dominated by local players, whether in gaming or non-gaming and even top grossing apps in the USA may likely be irrelevant in China.
Feel free to ask any more specific follow-up questions here and I will try to provide more insight.