Hi. For cole crops like the ones you mention, it is the leaves and stems that are eaten rather than the fruits, and pollination isn’t necessary for those to be produced. In fact, growers usually try to prevent these plants from flowering and producing seeds, since that pulls resources away from leaf and stem production. The same is true for root crops, such as carrots, radishes, and potatoes. Pollination is only necessary for plants that produce fruits, such as tomatoes, peppers, cucumbers, squash, raspberries, blueberries, etc. Row covers are a common way to prevent cole crop pests such as caterpillars. Here is a good fact sheet from the U. of New Hampshire that mentions that and other best practices for growing these vegetables. Ohio State U. also has specific fact sheets on cabbage, cauliflower, and broccoli. Note that both of those sources mention crop rotation as an important way to prevent disease. Crop rotation is not planting these crops in the same area of the garden in consecutive years (OSU recommends waiting several years between planting in the same area). Here is a nice, brief article on crop rotation from the U. of Wisconsin that you might find helpful.