Hi,

This is just a Vanilla Minecraft Stronghold schematic, just that, it has all the interior as barriers, so you can place it underground and then replace all the barriers with air like this with WorldEdit or FAWE (recommended):
/load schem MinecraftStronghold.schem
/paste -as
/replace barrier air
Also, if you want to edit it on air and then move it, it has diamond ore blocks on each corner... I decided to set them like that because if you just want to add it to your world, the diamond ore can blend in underground in the caves... idk...

the center of the schematic is the center of the end portal, so I suggest pasting it in Spectator mode or Creative mode...

The original schematic by Mikeoz28 is here: https://www.minecraft-schematics.com/schematic/6919/
the screenshot is taken from Mikeoz28, I will upload a new screenshot later for comparison, meanwhile you can check the 3D view...

I did some minor changes, like expanding a library, closing some holes with iron bars, changed a room, added 2 fountains, and the portal is not activated. I did not touch the chests, so don't forget to check those up...

If I do some other change I will update this...