aluminium foil closes the air directly buy the way of direct convention with the contents of the foil wrapped packages. Foil is not the good insulator of heat. So, it will not keep the things sow arm. How does aluminium foil keep things warm?
Metal conducts heat very efficiently. So the, foil by itself is poor insulation but an excellent barrier to hold in moisture which makes it useful to keep a baked potato moist but let some of the 350 degree heat escape and not scorch your tongue. In truth any metal will conduct heat from the higher point to the lower point. it is a poor insulator. But if it is put around a good insulated like fiberglass or ceramic material it will present an excellent barrier against air and moisture and make the insulation more effective.
Aluminum is reflective. Simply it reflects the thermal energy back at the object within it. This is why people put reflective covers under their windshields (bounces heat back out of the vehicle). This is why survival blankets are huge, thin, reflective sheets. This is why you can put foil in the windows of your house and save money on heating and cooling. Even chemists sometimes wrap glassware in foil to keep it warm inside. Ever notice that people are switching to metal roofs? It saves money because it reflects the heat rather than absorbing it like the dark shingles on many people's houses. It is all the same.
Metal easily conducts thermal energy, but reflective metals can bounce a lot of it back. If you are familiar with IR radiation and how greenhouses work, and how mirrors reflect light, I think you can add the two concepts together and get a decent idea of how reflective metals keep things warm.