It is ideal to make all the connections by tying, but sometimes this is complicated and unnecessary, and balloon decorators use various methods of connecting the balloons in addition to tying them together. For large areas, there are even templates in which balloons are just inserted and the wall is made. With smaller figures, usually these cheaper ones, to speed up and simplify the production, instead of complicated production of, for example, the nose on the head of a round balloon, it is easier to just stick the balloon. (It's even easier to draw it.)
Latex balloons can be glued together in a variety of ways. Double-sided adhesive tapes or adhesive dots already prepared for this purpose are often used, and there are liquid and soluble quick-drying adhesives that can be used to quickly and firmly glue balloons, like the wings of this simple angel sitting on a balloon pole.