Welcome to the game and to the forum. 
This might help with the transition.
Also this may help with understanding the game
https://www.grimdawn.com/guide/
You can get back to the other part of the map by simply putting down a personal riftgate and then clicking on any of the riftgates you’ve opened already.
Cairn is the name of the world - as we call Earth Earth. The game isn’t really divided into Acts, but flows continuously. However the term is used quite a lot to denote certain areas of the game so
Act 1 - up to where you kill Warden Kreig
Act 2 up to killing Cronley
Act 3 up to Darkvale Gate
Act 4 Asterkarn up to killing Loghorrean (end of the base game)
Act 5 Reaching Lone Watch
Act 6 Malmouth and killing Theodin Marcell (end of Ashes of Malmouth expansion)
Act 7 the whole of the Forgotten Gods area