This may be a thread for people with more experience, but I’ll share my thoughts for what it matters. When I got my current fellow he had a tendency to spook, but over time I began to notice two things, he spooked when I was tense, even in the slightest. That little more pressure in the mouth, tension in the shoulders or seat. I also got good and picking up when he started to get tense, which also meant that he was becoming more relaxed and giving me time to play the leader and set the tone before he would decide to take over. I also learned to really relax yet be attentive to him and the environment in general.
Much of it came from just seat time, taking him out a lot into different situations (we trailer to lessons, conditioning), not always riding, doing a fair amount of groundwork to establish a sense of “I’m the leader”. I remember when we went to the beach, I hand walked him to the tide line, got him so having the water touch was not a freaky moment and thought, we got this. I jump on, ride him back to the tide line and it was like he never saw it before. But, the idea that I worked with him on the ground, I feel, made it less stressful when now I was riding.
At home they get 24/7 time in pastures with various things passing by. I also think that helps reduce the spook since they connect a noise or something going by as not a threat. If he does spook when I ride, and it happens, I will work with him right there, almost like teaching them a drop bank or water entry. Get him to step forward, no backwards, till he can put his nose on it and then look at me like “What, didn’t scare me a bit”. That takes patience. Something else I learned.
I think spooking can be trained down with confidence, controlled stimuli, and lots of pasture time with calm horses.