1 In Singleplayer place some diamond blocks or anything, can be a shulker box full of anything

2 open Lan on the world

3 open the same world in another instance that you opened in lan, but dont open in lan this time

4 on the first instance you can now mine blocks that you placed in world, or take things out of shulker

5 close first the world on second instance then close the Lan one

6 when you open your world you should have both blocks in your inventory and in world
Edit: just realised that you dont need to open it in LAN, you simply open the same world in two instances, modify one, close second, then close first, and then after you open the world on the side where you modified it, the world is from the second instance but inventory is from the first