Yes, you can open the bottle opening.
The double-walled bottle with vacuum inside is the inner tank, and the outer cover is made of a heat-insulating container, which is a common household item.
The concept of a thermos bottle is simple: the bottle has an inner wall and an outer wall; the walls are vacuumed. Thermal conduction and convection are not possible in a vacuum, and the plated metal reflective layer can reflect thermal radiation. Therefore, any liquid poured into the bottle can maintain its original temperature for a long period of time