Contrails are clouds that form behind airplanes when conditions are right. Burning jet fuel creates hot moist air that shoot out the back of the engine, and also a fair amount of "soot" particles. When that warm moist air hits the very cold air around the plane, it chills, and condensation forms -- and you get a long thin line of cloud. As the plane moves on, the contrail (condensation trail) cloud might evaporate again quickly, or might hang around, or might spread out.
Can contrails persist? Yes, of course, because they are clouds and clouds can persist. Can they spread? Yes, clouds can spread, too. Do they form some days but not others? Yeah, clouds do that. Might there sometimes be just one, and other times the sky is full of them? Yes, that is a thing clouds do, and contrails are clouds. Can a contrail appear bright white, while another is dark and grey against the sky? Yes, like clouds, they can appear both ways, depending on conditions. Can they have breaks and gaps in them? Sure, this is a thing clouds do, even clouds that are not contrails. Are they sometimes scattered widely, while on other days the sky is full of them? Yeah, like other clouds, they can do that. They can do anything that clouds can do… because they are clouds.
All of the features that supposedly "prove" that this line in the sky is not a contrail, and so must be a chemtrail, are features that contrails can have, because they are clouds and clouds do those things,
Some people who want us to live our life in fear of nothing are trying to make us scared of clouds. People who are afraid are easier to mislead, easier to con, easier to control. People who are spreading baseless fear do not have our best interests at heart.