why is thing urgent bro? manchhe nai maryo ki k ho?

by the way it is CONTRAIL.

Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft. The exhaust of an aircraft contains both gas (vapor) and solid particles.

science-edu.larc.nasa.gov/contrail-edu/science.php

Those are made by big airplanes and are called contrail.

jerry bro nepal gayera sanchaikai boulayecha. aau BRO US feri bapas. ek fera ko jindagi ho bro. u should live the way u want it. not how others expect u to live.

Nas,
Those youtube videos explanation represents no more than some stupid conspiracy theory !
Kaji sahab has already explained the mechanism of formation of contrails.
As per the size and short lived vs persistent contrails - multiple factors come into play.
Different altitudes have different pressure, temperature, density, humidity etc. - thereby atmospheric conditions dictate how soon or fast these ice-crystals dissipate. Not to mention the amount released by the aircraft.
Simple experiment can be designed to verify this -
Heat water in a pressure cooker until it's boiling, and when the water boils do following splits:
1) Let the steam vent while the cooker is still on the burner.
2) Let the steam vent while holding the cooker inside the freeze [not freezer].

You will notice the difference is the amount of 'cloudy vapor' formation between the splits. By all means call it a 'chemtrail' after all it's made of dihydrogen mono-oxide, which is a chemical !

It might be some Indian fighter planes enjoying free sky of Nepal.