Fascinating: Once you put the lid on the jar, you don't need to add any water or fresh air!
PLEASE DON'T PLACE LIVING SNAILS IN THEM - THIS IS AN EMPTY SHELL
HOW DOES IT WORK?
Much like on a big scale a mini water cycle establishes. Water from the dirt vaporizes due sunlight and condensates on the cold glass, coming back down as droplets.
The reason why you don't need to add fresh air is because the plants both convert O2 to CO2 and CO2 to O2. The only thing they need from outside is a bit of sunlight.
Avoid too much sunlight, though - it will cook the plants! Instead place it in a shady spot.
Add very few water in the jar before you close it - I lost some terrariums to mold and moist.
My oldest terrarium so far is now about 6 months old. Never opened, still a fresh and green world.
Oh, and don't place living snails in them - that's only an empty shell in the video :D
P.S.: This tutorial covers the absolute basics of creating such a tiny world. If you want to learn more about this topic I can suggest the book Terrariums Reimagined by Kat Geiger :)