“Putting the filter in the HVAC is enough.”
No, you must run the HVAC system to get the benefit of the filter
“Filters work well when they are simply slid into place.”
This can be sufficient, but it is important to be sure the filter is sealed into place. If the air goes around the filter (called leakage or bypass) it will not get cleaned.
“Filters remove everything from the air.”
In fact, they only work as well as the MERV specification. In addition, particle filters don’t remove volatile organic compounds (VOCs) or ozone.
“That they will last forever without being replaced”
Different MERV filter require different change intervals and range from 30 days to 6mos.
“HEPA and MERV filters don’t catch particles below 0.3 μm.”
Filters get more efficient as particles get both bigger and smaller compared to the filter’s most penetrating particle size (MPPS), usually near 0.3 μm.