Notice that both my client with intellectual developmental disabilities (IDD) and the baby have both learned how to stabilize the container used for insertion/removal tasks. Babies develop these skills between 12 and 18 months and refine them by inserting or removing items that vary in size, shape, weight, length and firmness.
I love how the baby is using a box with a large opening that has a pink border to create color contrast. Parents can gradually decrease the size of the opening so that the older child is grasping bottles with small openings to insert objects.
Inserting laces, string or long strips of fabric are extra challenging since the child needs to learn how to manipulate and push near the bottle opening. Some of my clients hold such long materials distally so that they dangle near the opening and require training to grasp and push effectively so that the object is inserted. Teach this skill by offering fabric strips that are increasingly longer and longer- gradually increasing the challenge but keeping the task still achievable.