What is a game level?

Question

Swift

I am the very beginner igrodel, while I'm just having fun, I ran, like many, to study the newly minted Swift. While I like everything, I am writing my game, and now the question arose - what is a game level.
How do you create a game that has levels? Does each level have its own .swift file in which objects are placed? or is it a binary file that is somehow cleverly created and then parsed in the game scene?
It is not necessary for me to explain everything here, links will be enough (only not 500 pages of books, but something relatively specific).
Thank you!