instances, level map and replication

I need some information to get a better understanding of the subject, any information or knowledge will be greatly appreciated!

1 - Instances: Is it possible… well… it is possible… actually, what do I need to keep in mind or what information do I need to have an idea of how to create an instance… for example in online games when you are going to do a mission and you are teleported to a cave/area where only you are there (instanced quest) i.e., the server created an instance just for you from a map and put you there… what do I need to know to make this happen? If anyone knows a video or tutorial it would be very good… or any information…

2 - Let’s say I have 2 maps in the project, map 1 and 2, if I start on map 1 and load level of map 2, will this loaded level 2 have server replication? If another person also loads this level 2, will this player appear to me in the level 2 that I am in? Or does this need to be done with some kind of instance system?

These doubts are related to mechanics of changing maps like in the games Cabal Online, MU Online, Ragnarok Online and Albion Online, where the player is on a map and can use a teleport at the end of the map to go to a more difficult map with stronger monsters…

According to your experience and knowledge, in this type of game where there are several biomes/maps that the player can explore, would it be better to make each biome(map) on a different map file and then load them, or to make everything on a single map to form a giant map with several biomes(piece of map) where the player is just teleported to the coordinate of that biome(piece of map)?

I appreciate in advance for any help you can provide!