They're coming from outside. They establish routes that are many meters long through all available seams and cracks. Use Amdro ant bait (not 'blocker') in the spring around the structure perimeter when temps get above 10C/50F, avoiding wet weather. They carry the bait back to the nest and wipe it out. Follow the directions scrupulously and be patient; it will take a week. If you have a lawn keep it thick and healthy; ants usually prefer bare ground for nests. It's possible, with good landscaping and aggressive use of a caulking gun (assuming the structure is otherwise in good condition,) to keep them out, but that is often not feasible, and some ants are more industrious than others.