There are studies on AQ and classroom performance[1][2] that seem to arrive at similar conclusions. Obviously there can be conflicts of interest if the backers manufacture air filters, but I don't see any disclaimers on those, at least.
As my family of 5 wakes up, the CO2 comes down with them as their bedroom doors open (CO2 is heavier than air), and the CO2 level spikes downstairs - and can take a couple hours after opening the windows and doors to get down to the outside baseline CO2 level of about 420 ppm.
Here is a visualization where I did not open the windows right away, and had to close again due to the heat: https://i.imgur.com/1alDAfB.png
On summer days, my servers don't love the heat and humidity of outside air, otherwise I've learned to keep the windows open all day. I do seem to notice a difference in how I feel if the level goes over 1000ppm, but that's really hard to quantify.
Yes, unfortunately houseplants don't do much for air quality[1]. But they do look nice, and they can be satisfying to take care of (unless you kill them, that is).
1: https://www.edworkingpapers.com/ai20-188 2: https://pennstate.pure.elsevier.com/en/publications/the-rela...