Once you beat God of War Ragnarok, or at least get towards the end, you'll want to return to places you visited previously to complete side quests and tidy up all of the collectibles. You'll want to get back to Jarnsmida Pitmines and Applecore in God of War Ragnarok to find some things, but doing so isn't very easy.
As you'll know, Jarnsmida Pitmines and The Applecore are in Svartalfheim, and you'll have been there as part of the early story, in which you save Tyr from Odin's prison.
How to get back to Jarnsmida Pitmines and Applecore in God of War Ragnarok
First of all, travel to Svard Sands in the Aurvangar Wetlands in Svartalfheim. There, you'll need to head across the raft you can pull, which you used with Tyr previously.
Once you're on the small island, head to the left around the rocks and you'll spot a lift that can take you down into the mines.
From here, you'll notice on God of War Ragnarok's map that there's a big line going from the Svard Sands to the Jarnsmida Pitmines, through The Applecore. That line is the main mine cart track in the mines. While you can't follow it exactly, the best thing to do is stay as close to it as possible.
Once you've taken the lift down, keep following the path, grabbing the secrets along the way, You'll soon see the option to use the Draupnir Spear to climb up a ledge. That means you can't progress any further unless you've unlocked the third weapon in Kratos' arsenal.
After reaching the top of that ledge, turn left and swing across the gap, going down into the arena, where you'll come up against a mini boss.
After defeating the enemy, who isn't too difficult, climb through the gap in the arena and go through the next door you find.
Then, rather than doing the lift and water puzzle in reverse, shimmy along the ledge just to the right to bypass the puzzle. That'll take you to the Applecore, which is the big circular structure at the centre of the mine. To get out to the Jarnsmida Pitmines, you need to follow it round and take the final exit on the right, before the end of the path.
Jump down there and squeeze under the water way, climbing up the wall you'll then come across. Then, go through the door on the left and use your Draupnir Spear to swing across to the next section.
You'll then unlock a new Mystic Gateway, which'll make returning easier in the future, and you need to head left after it, going down through the hole on the left of that next room.
If you continue along that path, you'll make it our into the Jarnsmida Pitmines, where you'll find a new puzzle and some side quest objectives to complete.