I am actually curious about the answer to this as well.
As far as building a portal and using the vanilla classes to help for 1.15.2, I have found the following vanilla classes:
FlintAndSteelitem.java
NetherPortalBlock.java
Teleporter.java
and possibly the tile-entity classes for the end dimension but they don't have the same mechanics as the nether. At this point I'm just curious where the vanilla code is for teleporting from nether to overworld is. The teleport class really only has methods for building the portal and correctly positioning the player it seems.
I'm not an expert in Minecraft modding but I've been looking online for any sort of documentation on this and could only find this thread on forge. I have a dimension created and all it's missing is the portal. Hopefully it's an easy fix!