Few questions about how BP system works, esp gamemodes


I’m a bit confused concerning Blueprints and especially Gamemodes.

Are gamemodes blueprints used to handle the game logic? For example, at the beggining of the game, it will load up the savegames, etc. Is that where I should put everything which is not related to a particular actor ? (class BPs)
Because on the documentations it says it’s mostly used for the multiplayer interaction, but then where do I execute all my “main()” code?

Also, I see in some videos people create blueprints for specific use and drop them in the middle of their game. For example, they do some game logic code in their blueprints, don’t create any actors in them and drop them in their game, showing the little dinosaur face. Is it used to be executed at the beggining of the game too, like gamemodes?

I think the way how Blueprints works with each others is a bit confusing, could anyone explain me a little better?

Thank you!