Adding food colouring to a bowl of water. See how the concentration is greatest near the source of food colouring.
This is equivalent to an infected person releasing virus into an unvented space 1/n

Over time the food colouring mixes, equivalent to virus building up inside. Note I didn't continue to add colouring, in reality an infected individual would continue to add virus, increasing the concentration

If I keep the tap on and loosen the plug this is similar to ventilating a space, adding outside air and removing indoor air. Concentration is still highest next to source, it dissipates quicker, and overall concentration is less

This is over simplistic, but hope it shows the importance of both social distancing and ventilation indoors. Face coverings would help reduce the viral load (and maybe inhaled dose). Reduced exposure time will reduce inhaled dose

If I take some of the contaminated water and add to a fresh bowl, similar to air moving from contaminated space to an adjacent space the food colouring is difficult to see as it is so diluted. Note it is not a zero amount, but is much less than a room with pollutant source

Further advice and guidance in cibse covid ventilation v4

https://www.cibse.org/coronavirus-covid-19/emerging-from-lockdown