Those cloud seeding programs are expensive and require dedicated hardware.
They are not installed on commercial passenger airlines. If they were, that would mean a huge number of aircraft maintenance workers and ground crew know about it, and are keeping it secret from the public - a conspiracy. Where are the chemicals stored at the airport? Are the fire/safety teams also in on the conspiracy?
If not on commercial passenger airlines, then which ones? Are there fleets of planes which are not on publicly available flight trackers? Or are a lot of those flights lying about their cargo and mission? If so, that's a conspiracy.
Those weather modification programs are not done in clear skies. The video you linked to earlier makes that very explicit. All cloud seeding does it change the timing of where rain falls. It doesn't work without a lot of water already in the air.
For rains to not fall, when it would have without active intervention, means the water had to be forced out before reaching Spain - doing it over Spain would be worthless. So pictures of clear-sky "chemtrails" over Spain can't be weather modification keeping rain out of Spain. Where are these flights based? Where do they travel? What conspiracy keeps us from knowing this about Europe, when we know about China's efforts?
That's why these are called 'conspiracy theories'. They require a large number of people to conspire to keep something secret, and have at best only weak circumstantial evidence to support it. ("People can modify the weather, so the droughts in Spain must be caused by people modifying the weather" needs a few more underpants gnomes worked out.)
Thing is, we know that contrails modify the climate, without a doubt. 9/11's flight shutdown made that analysis possible. "9/11 study: Air traffic affects climate" at https://edition.cnn.com/2002/TECH/science/08/07/contrails.cl... or https://www.nature.com/articles/418601a if you want the publication. It helped reduce the daily range in daytime highs and nighttime lows.
Which makes continued flights a form of weather modification right there, just like dumping massive amounts of CO2 in the air is another form of global weather modification.
But notice that those are both side-effects of a much more profitable business.
Who benefits in keeping the rain out of Spain, and how do they keep those activities hidden so well? Oh, right - a global conspiracy.
What is the right solution if people are extremely anxious for strongly illegitimate reasons?