How can I get back once I’ve gone out
You get back in through the portal you exited it through. That’s on route 5 up a waterfall. So you’ll need to go to Holon first to get the HM item for that.
This will work only after you have obtained the waterfall hm (magic carpet) and ability to rock climb outside of battle.
Take the east exit from Koril town and use waterfall on the bigger waterfall of the 2 present. Then enter the the cave. You’ll figure it out then.