All good and correct answers here. All I can do is re-tell it in a different way.
MTL (Mouth To Lung) vapes generally produce less vapor. Usually these devices have smaller batteries, and are easy to fit in your pocket. Because they produce less vapor, They usually up the nicotine, Larger direct lung systems open up the airflow and produce more vapor, are larger, and more customizable to your liking. Because of the greater volume of vapor, the nicotine level is drastically reduced.
So, If you see someone blowing some massive clouds, that doesn't mean they're vaping more nicotine. For example, the Milligrams of nicotine per Millilitre of juice will be real high on the smaller setups... like Juul pods. those can have like 48 MG/ML juice in them while the cloud chasing setups generally have 3 MG/ML or lower (and it's common for them to vape zero nic).
So, Just as an example, If a juul pod uses about 1ml of juice and someone uses one pod per day, they are vaping about 48mg of juice per day. However, a cloud chasing setup might go through 5 MG of juice. So if you put the juice from a Juul pod into a cloud chasing setup, they will immediately notice the higher nicotine.... and they wouldn't be able to vape it...and if they did, instead of going through 48 MG/day, they would be vaping about 300mg/day (If they forced them selves).
I hope that helps you understand a bit more about it.