Comments on the buildings (I'm an engineer with training and experience in this area and spent a couple years in China)
As a general rule, Chinese buildings are shiny on the outside and rotten on the inside. It is very common for them to use too little rebar, of low quality, and poorly assembled. Often there are serious structural design flaws which are simply glossed over in construction. Vertical walls are almost always unreinforced masonry (red bricks), which is surprising because China is famous for earthquakes. Unreinforced masonry, especially dozens of stories high, is a death trap in earthquakes. Every Chinese city I have seen would be a death trap in an earthquake. In general I think the buildings will not last nearly as long structurally as you would expect based on buildings in the US, and in practice the shine becomes rust very fast, not sure if they have improved on that recently. None of these buildings, or any city in China, has water that is safe to drink out of the tap. In general I feel that China and the US have different standards for public infrastructure: US infrastructure looks terrible but works well, lasts a long time, and is cost effective. China has basically the opposite standard.
Some of the recent projects sponsored directly by the emperor are a little better, they seem to have been constructed decently and according to plan, though even those I thought had overly slender columns and too little rebar compared to what I would expect, and construction quality was at best what I would call average. In general impressive infrastructure is sponsored by the government to show its strength and prosperity. There are some truly awesome projects out there, but most of them don't seem to have gone through any kind of cost-benefit analysis. While admitting that blowing smoke out of lifted trucks and buying and sending cheap plastic trinkets to the landfill is not a better use of resources, the reason other countries don't do infrastructure like this is because it is a waste of money. And also it would be very difficult for us to procure the property. A lot of villagers (I mean really a lot) are pushed out of their homes and into planned developments by these. Having visited a couple, I can say that they feel very unnatural and somewhat dystopian. It would be like if someone built a 30 story apartment complex in the suburbs, with massive sweeping manicured roads and garden features with lights, and no shops around, and the residents could barely afford cars, public transportation was vital but sporadic, and the water wasn't safe to drink, and the building would start to fall apart in 30 years, and they couldn't get another one, and their entire life experience was living as farmers in traditional single story village houses.
I will say this: the Chinese have become really good recently at applying a great looking finish to the inside and outside of a building. Definitely a higher standard than all but the very very best in the US. And that goes for common apartments as well as government infrastructure exhibits. However, I do think that much of it is cheaply done and will age very quickly.
Slave labor: no. Not everything, but pretty much everything, in China is made by people who want to do it and are free to leave. And they frequently leave. From what I hear Chinese labor turnover outside government jobs is very high. They have awful safety standards sure, but people are mostly happy to leave their old lives scratching the ground with hand tools for literal peanuts to get a high paying job in construction or industry. My FIL has been on a number of construction jobs as a crew boss (nothing like the buildings in the video) and no rumors of slave labor at all have reached my ears. Some pretty terrible accidents though. He had to stop work for a year or two about 5 years ago because of PTSD from one of them.
Comments on investments: I would not invest more in China than its global capitalization weight. Like Putin, the government has been on a real "everything we say and think is right, and everyone else is wrong and they don't matter" streak for the past decade. It has been becoming really obvious especially over the past few years.