High pass or low pass? Many instruments have a low pass filter. Very few have a high pass filter.
The instrument is pretty crappy - it has basically no calibration or, AIUI, long term stability. So you can’t really use it to characterize the chemical content of the air. What you can do is observe changes. I’m guessing the manufacturer wanted to avoid people getting confused by the absurd raw reasons, so they came up with a formula called “VOC Index” that HPFs the data. It will average 100 IIRC. You’re supposed to use it to notice events that cause changes - if it hits 170 every day at 2 PM them you can guess that something’s happening at 2 PM.
None of this is in the chip. It’s implemented by the driver. There’s an implementation in esphome, for example.
correct, that's not VOC sensors do. you need specific sensors for different gases for that. All VOC sensors are metal-oxide (MOx) sensors, i suggest reading on their supposed applications and uses.
But the actual behavior, at least for the SGP4x series, is so outrageously unstable and path dependent that you still can’t learn the value if that function of the gas concentrations. At best you get changes on timescales of minutes to a few hours but not on timescales of days.
again, correct. That's how they're supposed to work. If the air is very slowly getting worse, MOx sensors might not catch it.
That being said, what "very slowly" means depends on the sensor. For example, for Bosch BME680 (very good with 'organics'), you can set the tracking interval between 4 and 28 days (i use 28). Sensirion SEN66 (very good with 'chemicals') can have a tracking interval of up to 1000 hours.