Here is not a very good answer to your question, but think about how you know how to walk. It's something that is not entirely conscious. You legs just move bet

Here is not a very good answer to your question, but think about how you know how to walk. It's something that is not entirely conscious. You legs just move between places that you desire to go without any thought to the angle of your knee or tension in your gluts. You feel unbalanced when you center of gravity is too far forward despite having no conscious understanding of where exactly your center of gravity is in the first place. Spiders likely have a similar feeling but even further refined. They don't know how to build a truss, but they do end up building analogous structures because in their tiny bug brains it just feels right. This is the same way a human knows they are more stable with their legs spread apart and knees slightly bent. Part of that certainly comes from learning but a lot of it is built into our biology too.