Fresh air systems are what you are looking for. They purify and bring in outside air (be warned, they lower HVAC efficiency), a lot of them with a high enough CFM to pressurize your house to, exhausting all of the air inside outward so that the leaky seals aren't an issue.

The problem is that these don't fit into most Chinese apartment rentals. Landlords could make some big bank by installing an HVAC, but they are missing even in new buildings where 90sqm apartments are going for more than a million bucks.

Yep. My little apartment (400m²) in a low-rise in Canada, built just five years ago, has no in-unit ventilation whatsoever. If I want a cross-breeze (and I always want a cross-breeze), I have to open the outside windows and the door to the hallway, and let a ton of dust and pollen flow into the unit from outside. I have no idea who thought building an apartment building today like this made sense, but it seems to be a common perspective.

400 sqm is a "little" apartment? Sheesh, first-world problems :)

That's why HRV systems exist.

Keep your environment's air clean while retaining the heat plus circulating fresh oxygen.

I intend to get a decentralized HRV like Lunos for my passive Tiny House. Any problem regular houses have with air quality, will be multiplied in a Tiny House without proper precaution.

Modern insulated houses (think Passive houses) have automatic heat recovering ventilation because everything is sealed. Many heat recovering ventilation systems that have an air purifier integrated. Many also vary ventilation automatically depending on the inside air quality.

More plants in your house. It makes a surprising difference.

I open the windows for 20 mins, twice per day. The rest of the time, the windows are closed and air purifiers are on.