map transition is done through scripting - specifically the <TRA command. you assign an event number to a door (or whatever entity you're using, it can be pretty much anything), make sure it's interactable, open the script tab, and under the door's event number write '<TRA' plus its arguments - i believe it's <TRA[number of the map you're going to]:[event that will run once you enter the new map (the 0090 events in vanilla)]:[x coord]:[y coord].
keep in mind that:
- all argument numbers must be 4 digits long (you can add 0 to the front to buffer it out)
- event numbers also have to be 4 digits long and start with a #
- event numbers also also have to be in numerical order in the script tab
- there's a few more commands usually used in map transition - <FAO (fades the screen out), for example, but you can always check vanilla scripts for reference
hope this helps!