Just to add on to this:

Think of a thermos like normal cup with a lid and a blanket wrapped around it.

The lid prevents hot air from escaping, hot air escaping makes room for more hot air and steam that then escapes, and the lid basically just keeps heat from escaping through that series of events.

The blanket (usually a gap of air between two layers of metal, but sometimes glass, sometimes styrofoam) does what blankets are made to do and doesn't transfer heat, so the heat inside the cup doesn't leak out through the blanket much.

This is what keeps your beverage hot - the fact that all the ways heat usually leaks out of it are sealed up well, so it takes a lot longer for the heat to leave the liquid. Same thing with cold beverages - the protective barriers of the thermos keep the heat outside of the cup from getting in.

This also means you can enhance any thermos or cooler with your own blankets. Literally wrapping a cooler in a blanket will keep the stuff inside cold much longer than any amount of engineering plastic into a layers of foam (which is basically how most coolers work - they use plastic foam as a blanket). Similarly, putting your thermos inside of a blanket will keep it better isolated as well. I have no idea how long you could expect to keep something hot, but with enough blanket you could conceivably get days out of it.