[Unity Beginner] Game design question on scene loading and scene transitioning

Hello Community!

I'm new to Unity, have built and deployed a couple of very simple games based on official/YouTube tutorials in which is common to use SceneManager to load scenes based on its buildIndex.

Now when playing commercial AAA games I notice how at any point I can bring a complex game menu (for example a world map, an inventory or skill/level-up screen) with videos, 3D models I can zoom, rotate, etc. And then transition back to gameplay really fast, seamlessly.

Question 1: In Unity is loading scenes is fast enough to go back and forth and reproduce this behavior? (again assuming game menu/screen is complex) or larger games use another method to load such scenes and register changes (for example I have met/discover new characters and quests, more skills and more items on inventory)

Question 2: Games like that the latest Zelda which let you explore big areas non-stop without loading screens, how can Unity reproduce this functionality? Assuming an open world game in Unity, where the whole world does not fit on memory and is not contained on a single scene. My guess would be physical triggers are placed on scene x such that scene Y loads but I'm not sure if that would allow the player to continue exploring without lag.... also, assuming some way of procedural loading of sections of an open world, how games like GTA V allow you to see landmarks of very far away places (which I guess that section is not loaded on memory yet)

I appreciate guys your time and and valuable response you can provide me. Thank you