As you wish! Making an RPG is complex business, and it probably pays off to familiarize yourself some more with GMS2 before jumping in at the deep end. The engine isn't going anywhere, so you can come back whenever you feel ready! ^__^
To answer your last few questions:
A lot of map objects - doors, NPCs, items - has their variables set via the new-in-GMS2 GUI variable system (double-click a trainer NPC and then click the "variables" button on the details window that pops up for an example).
The code that warps you to the starting map at the end of the intro is in script cc_intro_startgame (cc is "cutscene command"). You can get some examples of how to make cutscenes by checking out any cis_* script ("cutscene initialization script")